This is usually an issue with the stitch regulator or encoders. Here is a video that demonstrates the stitch regulator very well.
https://www.youtube.com/watch?v=pijhu8Cyew0
Check your encoder wheels. Are the skipped stitches going side to side (back encoder X) or for/backwards (side encoder Y ).
If they have rubber around it may need replacing, or the tracks simply need a good clean. These are issues I have with my Lizzie, & can be a real pain.