Going to the old book on sewing I got in a yard sale, it says about Skipping Stitches
Needle in backwards
Machine needs timing
Use correct needle
False timing, or poor needle insertion
Needle bent or burred or dull
Lint in upper or lower thread handling mechanism
Damaged Hook
Incorrect needle plate
Check spring
Poor Thread
Material not prewashed
Tension too tight
Handling material wrong==it suggests firm holding of fabrics
Burrs
I can go into further checking on these if none catches your eye and helps right away.