Back to my old book on sewing...under the title Skipping Stitches
Needle in backwards
Use correct needle
Machine needs timing
False timing, poor needle insertion
Needle bent or burred or dull
Lint in lower or upper thread handling mechanism
Damaged hook
Incorrect needle plate
Check spring
Poor thread
Material not prewashed?????
Tension too tight
Handling material
Burrs
If you think it may be one of these, PM me and I'll write out what the book says might help.