Frances, Check to make sure that you don't have a bad needle. If there is a slight burr on the point of the needle, it will cause this. Or if your needle isn't in straight.
If you have broken a needle, you may have a groove on the inside of the needle hole in your throat plate. You can take your plate off and buy emery string to run back and forth to smooth that out or you can buy a new plate. (look under your plate and there should be a number engraved on your plate to guide you into purchasing the right plate.
Check your bobbin to see if its wound evenly and that there aren't any grooves in the edges of your bobbin (if there is, throw it away).
Lean your machine back and run your finger along the outer edge of the shuttle to see if there is a rough edge that might be catching the thread as it revolves around. If there is, buy a piece of fine emery paper at a hardware store and buff it smooth
Are you using smooth thread and a big enough needle? If your needle eye is too small for the thread or the needle is too small to punch a big enough hole in the material for the thread to pass through easily, it will sometimes cause skips like this.
As a last thought, try a different foot. If your foot doesn't hold the material tight to the feed dogs while you are sewing, the material will 'follow' the needle up and down and cause skips.
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