Loopy underside on triple cover hem - Elna 945 Computerized Sewing Machine

Not knowing your machine make/model, this is a guess. Be sure to Always Raise the presser foot when threading the top thread. This makes sure the thread is fully seated in the tension disk.

Rethread the machine. Its almost always caused by poor quality threads or misthreading.

try adjusting the top thread tension one number or letter higher
and sew on a scrap piece of fabric till the stitches look normal.

Tension ajustment for the underside thread might solve the problem

You didn't give the brand, but if it's any of the older, non computerized machines, your problem is most likley in the tensions.
Loopies (tangles or nests of thread) on the bottom means upper tension too loose (or bobbin too tight).
Let's do the bobbin first...once it is set...99.9% of loopies on the underside are a problem with upper tension.
This is for bobbins that load on the front/side of the machine.. Take out the bobbin case...check the area (hook/race area) that the bobbin goes into for lint, tiny thread pieces, etc.). Now, most of this type of bobbin loads into the case with the thread going counter clockwise (when you are looking at the open side of the case), insert the bobbin in the case, and pull it through the hole/slott in the side of the bobbin case. To test for proper tension:
Lay the bobbin case in the palm of one hand, pull on the thread coming off the bobbin case. The bobbin will gently lift off your hand & if you wiggle the thread, the bobbin case should slowly drop about an inch at a time. If it's too tight, loosen the Larger (closest to where the thread comes out) screw on the side of the bobbin case about 1/16 of a turn...tiny bit...retest. Once that is done try a test stitchout. Remember to hold onto the bobbin & top thread for first few stitches.

If you still get loopies on the bottom...tighten the upper tension...it should be set between 4-5 for regular sewing.
Make sure the needle is in properly.

For a drop in bobbin...adjusting that tension is harder. You take the face plate (needle plate, the plate covering the sewing area of the machine) off and remove the bobbin case...First...look at the case in the machine...there is a little lip sticking out, usually at the bottom right corner of the open area (closest to you)...this is where you need to have the little lip when you replace the bobbin case.
Remove the case...check under it and inside the machine for lint/dust/threads & using a small paint brush (I use a childs tooth brush sometimes), clean the lint out. Now check the bobbin case for lint, especially where the thread fits..look at the inside of the case, you will see a little flat metal thing with a slott in it...make sure no lint in that slott. I use a pin to remove lint from there. Reinsert the bobbin case, rethread it all, (bobbin goes counterclockwise). Put the faceplate back on & test the stitches...still loopies with the upper set to between 4-5? Then you will remove the faceplate, remove the bobbin case & loosen the screw on the outside about 1/16 of a turn (these are usually covered in a colored coating) and retest.
I know that most home machine makers say not to adjust the bobbin..but that was back when you had one size thread...now we have many, many sizes and types of thread, so adjusting the bobbin may be needed sometimes.
Rule of thumb is:
loopies on underside...tighten upper tension
loopies on top...loosen upper tension.
But, like I said...with all the new threads..sometimes you have to adjust that bobbin.

Not unless the model you are using has more than one height adjustment. This would be addressed in the instruction manual for the machine.

If you raise the heigth of the presser foot too much, the tension control will open and the thread will not form a proper stitch - you will get a loopy stitch that will not hold.

Intersecting seams where you may have 8 layers of fabric can be flatten somewhat with a nylon hammer, or lay a cloth over the seam and use a regular hammer.

I use this when hemming jeans - the flat-feld seams are super thick when turned under for hemming.

Use a longer stitch length when sewing thick fabrics.

Make a sample, replicating what you will be sewing to test techniques and adjustments to the machine before sewing your project.

loopy underside means upper tension and or threading is wrong

The first thing you can do is check the tension, sounds to me like it is too loose. Then be sure the bobbin is in and threaded correctly.

Hope this helps.

I found the manual on line and the other posts at fixya.com helped me solve the problem.

It's possible that there is foreign debris in the top tension assembly ,thread the machine and lower the presser foot and ensure that there is tension on the top thread