Poor stitch on underneath when sewing on material

ALWAYS RAISE the presser foot when threading the top thread. (You will need to remove the top thread from the machine and rethread from the beginning.)

For tension, start out with it set about the mid-point on the tension dial. Then tweak it from there. If top thread is showing on the bottom of the fabric, tighten the top tension. If bobbin thread is showing on the top of the fabric, loosen the top tension. It's ideal when both threads meet in the middle of the fabric.

Needle old or bent, incorrect size. Incorrect thread for material, packed with lint. The sewing foot is not on correctly or the needle is not pushed in high enough. Old thread. Hope this helps.

First thing to do is install a brand new needle.


Basic Maintenance Tension Skipped Stitches

What to Do If Your Sewing Machine Is Skipping Stitches

10 reasons for skipped stitches

...

Your first step is to put a backer--stabilizer, pelon, or material underneath the area to be darned. You can use spray adhesive, or use an iron-on backer.

Generally, you should use the zigzag stitch that is several stitches--not the one that does one stitch in one direction and the next in the opposite direction. Make sure that the stitch is wide enough to cover the area next to the tear and hold it securely. It is a good idea to stitch over the tear several times.

Skipped stitches is usually indication that needle is blunt or burred.
Or if you are using a sharp needle on knit fabrics it may also skip a little.

Not sure what you mean by "change weight of material" - do you mean you are sewing applique and going from one fabric weight onto another? or different sewing projects?

It is a good idea to change your needle for each new project (4-6 hours sewing time max) and choose the right type of needle and size for your project.

However, the jamming part sounds more like thread tension issues, especially if the jamming is happening underneath from the bobbin. If stitching is loose underneath, then top tension is the problem and visa versa.

Recheck your threading path, top and bottom tensions and that you have good quality thread. Change the needle, rewind a fresh bobbin, thread up and try again.

There is really good info on www.sewing.about.com or I refer to my Singer Sewing Machine textbook at lot too. Good luck.

Your tension is wrong and you need to check your stitch length.

Try a different needle, matched to the weight of your fabric.

Frist make sure that the bobbin is in correctly I know it sounds silly but check I have at least 2 machines a week or more come in and this is all that is wrong with them. if that is ok then check the bobbin case the black thing the bobbin sets in and see if it is nicked or damaged that is what normally causes this problem

It sounds like your tensions are out of adjustment. Your upper tension should be between 3-5 and make sure the tension in your bobbin case is not too tight as well. Pull the thread through the bobbin case and make sure it comes out with a small amount of resistance. If no resistance, tighten the tension just a bit, if too much resistance, loosen it just a small bit at a time.

Sounds like a bottom tension problem, to be sure I would have to have a look, I would suggest you have it looked at. Not sure about the materials you are sewing and have serious doubts as to whether the machine you are using is up for the job and will give you a satisfactory finish. It is possible a "walking foot" machine is what you need.