What should I try next?

For FMQ, make sure you have a darning or FMQ foot for your machine. You must use this foot because it is shorter than other presser feet so it does not press down on the fabric. You need to be able to move the fabric freely under the presser foot. When threading your machine, ALWAYS RAISE the presser foot so the thread will seat properly in the tension disk. The presser foot is ALWAYS lowered when FMQing or the tension will not work. To set the tension, test on some sample quilt sandwiches made with the same materials you will be quilting. Be sure to use a brand new quilting, microtex, or top stitch needle in your machine. Skipped stitches generally indicate the needle is dull, bent, or otherwise damaged. Test your stitching on the test sandwiches and adjust the top tension until the top and bobbin threads meet in the middle of the fabric (Increasing the top tension pulls the bobbin thread up. Loosening the top tension allows the bobbin thread to pull the top thread down.) Depending on your thread weight, you may need to adjust the bobbin tension, but do that ONLY if you cannot get the top thread to pull the bobbin thread up into the sandwich. (However, remember that if you adjust the bobbin case, it may not work properly when you go back to regular sewing.) I actually bought a separate bobbin case that I have adjusted only for FMQing on my machine and I use it only for that purpose.

Understanding Thread Tension on Your Sewing Machine

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Tension for stitching a quilt sandwich is tricky. It helps if you use the same color for top and bobbin thread,. If your quilting thread is not the same weight (either heavier or lighter) than thread used for regular sewing, then the bobbin tension may need to be adjusted. My quilting thread is lighter weight so I have a separate bobbin case whos tension has been adjusted specifically for quilting. The goal is have both threads meet in the middle of the sandwich. If the top thread is showing under the sandwich, you probably need to tighten the top tension so it will pull the bobbin thread up into the sandwich. Do the reverse if the bobbin thread shows on top.

It may also make a difference by changing the type and size needle. I tend to prefer a top-stitch needle. An embroidery or microtex may also work for you.

Machine Quilting Thread and Tension

Tension troubles on your quilting machine Try these tricks

5 Free Motion Quilting Problems and How to Fix Them

Quilting is a bit more finicky than regular sewing. Be sure to ALWAYS RAISE the presser foot when threading the top thread. FWIW, I use a separate bobbin case for FMQ that I have tightened the tension to keep the bobbin thread from coming to the top of the sandwich, but I tend to use a finer thread for quilting so adjusting the bobbin takes care of it. In your case, however, it sounds like the top thread is staying under the fabric. I would try tightening the top tension. Also, install a brand new needle--a top stitch, embroidery, or microtex needle seem to work well for me. Test well on a sandwich made of the same fabric and batting and get it adjusted well before sewing on your quilt. Sometimes sewing speed will affect stitches under the fabric, or even on top.

5 Free Motion Quilting Problems and How to Fix Them

Correcting Sewing Machine Tension for Free Motion Quilting NQC

LOOPS Tension or Speed

https://www.generations-quilt-patterns.com/machine-quilting-thread.html

https://www.generations-quilt-patterns.com/sewing-machine-tension.html

https://www.generations-quilt-patterns.com/machine-quilting-tension-problems.html

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Loosen your top tension. But, I usually have to tighten the bobbin tension too for quilting. That's why I have 2 bobbin cases. One is set for standard sewing and the other has the tension set for quilting...

Bobbin Tension How and Why Tension is Adjusted

Adjust the tension the same way as sewing regular projects. You want the top and bobbin threads to meet in the middle of the quilt sandwich. When quilting, I use a lighter weight thread so I have a separate bobbin case whose tension has been tightened specifically for quilting. For other sewing projects using a standard thread weight, I use a bobbin case that is factory set and then adjust the top thread tension for each project.

Could be several things.

First, install a brand new needle, perhaps a top-stitch needle will work better because it has a groove in the shaft that protects the thread. Make sure the needle is the appropriate size for the thread you are using--a needle eye that is too small will cause the thread to hang or shred. When the needle penetrates the sandwich, the top thread forms a loop. If the eye is too small, the thread will not come back up through the sandwich, leaving a loop under the fabric.

Remove the top thread from the machine.
ALWAYS RAISE the presser foot and rethread from the beginning.
Set the top tension to the midway point and retest your machine on a quilt sandwich made of the same fabric layers as your project.. Tension problems are more evident when sewing multiple layers of fabric, so tweak the tension until the top and bobbin threads meet in the middle of the sandwich.

I don't believe you will have to adjust the bobbin tension, but in case you do, be very careful. Mark the tension screw setting FIRST so you can reverse any changes you make. Bobbin adjustments are made in very SMALL increments. Because I tend to use a lighter weight thread for my quilting, I have a specific bobbin case that is adjusted for my quilt thread and I keep my other bobbin for regular sewing. Then I don't have to keep adjusting the bobbin tension when going from sewing a quilt to something else.

Some machines do better with certain brands of threads than others.

Adjusting Sewing Machine Tension to Prevent Problems

check the timing between the needle and rotating hook also check the teeth for possible lost of timing maybe low or high to much.

Suggest you check your bobbin tension, pull the bobbin thread and if you feel it too loose, most likely bobbin thread not threaded correctly into the bobbin case tension plate. Do refer to your manual if you have one.

Hi! The bobbin tension rarely needs to be adjusted. First, make sure you're using the appropriate size needle and thread for your fabric (I'd probably use either a 'denim' needle or a universal size 14) and also that the upper thread and bobbin are threaded absolutely correctly. These can throw off the tension! Also, take the time to clean out the bobbin area. That can throw your stitch off also.

If the stitch on the bottom is loose, so that you see the top threads, then the top tension is too loose. If the top stitch is too loose and you see the bobbin threads on top, then the top tension is too tight.

It helps to thread the machine with different color thread on top and in the bobbin, and then stitch an inch or so on the same type of fabric as you're using in your quilt. Check the stitch. If you need to adjust the top tension, make sure the pressure foot is down! Adjust a little, stitch a few inches, and check again. Repeat until you don't see any (or very little) bobbin thread on the top or top thread on the bottom.

If you go through all these steps, and the bobbin thread is still loose (and you've tightened up the top tension), then there is a tiny little screw on the side of the bobbin case that you can use to adjust the bobbin tension. Turn the screw just a little bit (less than a quarter turn), stitch, check, etc. Once the bobbin tension is set, you shouldn't ever have to change it.

Let me know if this helps, ok?

Happy sewing!

Try at least 2 new needles first (HLX5, preferably Schmetz). If same, look for any rough edges on the needle plate and feed dog cover.
Beyond that, this does not sound like you are doing anything wrong. You should take your machine in for adjustment by an authorized technician. You should have about 1/2 to 1 full turn of the upper tension knob between too loose/ugly on back and too tight/breaking on the top.