I am sewing swimsut material and the machine is skipping some stiches.

Timing has nothing to do with it. You have to have the correct size needle for both thread and material. Plus correct tension. Plus the correct thread for your material. Make sure both bobbin and needle thread are the same and make sure that the needle is not blunt and that the thread is not old and knotted. :-)

These may be helpful:

What to Do If Your Sewing Machine Is Skipping Stitches

10 reasons for skipped stitches

Basic Maintenance Tension Skipped Stitches

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Replace the needle it might be bend and check if it is the right needle for the work you are doing. You get for example stretch needles, denim needles, needle for thick/thin yarn, thick material ect. Using the wrong needle for the wrong material can cause skipped stiches especially if it did not do it before. Clean and oil the bobbin holder and make sure the bobbin is threaded correctly. Can also adjust top yarn tension as a last option.

Your top tension is based on the fabric thickness.. not just a number. Make a test sandwich of all the materials you are quilting with, cotton top, batting, lining, etc. Test your tension on that until you get it right.

The skipping is a symptom of a bad needle or the presser foot not having enough pressure on it. I'm not familiar with your machine so I don't know if it has the ability to add more pressure to the presser foot.

SKIPPED STITCHES
1.the needle is not inserted correctly
2.the needle is damaged
3.the wrong size needle is being used
4.the foot is not attached correctly
if these suggestions don't help the the machine could be out of
time & the machine would need to be taken to a sewing machine
shop to be checked

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.

You need a heavy duty needle #16 or #18 to sew leather and teflon presser foot is helpful

(A)Put in a new needle and rethread from the beginning. Make sure the take-up hook is threaded. Also, (B) clean lint out - sewing machines have very small margin for error when lint pushes everything out of alignment. Mostly, it sounds like your thread is stuck somewhere, which can even be at the spool, so (A).

Change your needle and use a ball point.

Replace your sharp needle, code 2020, with a ball point needle, code 2045. This usually solves the problem. You can also put "Stitch and Tear" at the bottom to help stabilize the material. If you can do not have Stitch and Tear, try a piece of tissue paper for the short term.