Are you using an old needle?
Make this a rule for yourself: new sewing project, new needle. Sewing machine needles can become dull after about 7 hours of sewing, and a needle that's dull, bent, or simply old can easily make your stitches skip. At the same time, make sure that your needle is in all the way, if you don't loosen the screw all the way before you insert, then your needle may by to low. Not only can this make stitches skip, it can actually damage your sewing machine.
Have you tried to re-thread the machine and wind the bobbin?
Most times when your machine is acting up, a simple re-thread of your sewing machine is the answer! Make sure that the thread is in the tension disk and the spool is placed correctly on the machine. Also, double-check to see if the bobbin is in the correct way and threaded tightly. Sometimes, a poorly wound bobbin is the culprit for skipped stitches. Good quality thread also plays a part in nice and even stitching. You really get what you pay for so don't skimp on thread if you have a sewing machine skipping stitches.
Have you adjusted the tension settings?
If you have done all of the above and are still experiencing skipped stitches, then you may have some tension issues. Adjust the tension dial in small increments after each test sew, and also refer to your sewing machine manual for recommended tension settings for your fabric. If you have a bobbin with a metal case, you may need to also tweak the tension for your bobbin. This can be done with a small flat head screw driver turning the tiny screw on the outside of the case.
Are you using the correct needle for your chosen fabric?
Sometimes, it is just as simple as you are not using the correct sewing machine needle for your machine! Sure, you may have swapped out an old one for a new one, but if it is not the correct type, it will still skip stitches. This is most common found when working with knit fabrics. When you sew with jersey, spandex, ribbing you must use a stretch/ball point sewing machine needle. To learn more about sewing knits on a regular sewing machine, register for this online course on Sewing Knits Without a Serger! Get 30% off registration with code MSKNITS30. There, you can learn more about troubleshooting skipped stitches when working with knit fabrics and much more!
If you have tried all of the above and are still experiencing skipped stitches, you may need to take you machine in for a tune-up.