Here’s a short list to consider if you’re having issues with stitches skipping.
Wrong thread for the application.
Quality defects in the thread.
Improper needle / thread size relationship.
Worn or defective thread guides or eyelets.
Improper threading.
Excessive machine thread tension.
Defective needle or improper positioning of the needle.
Needle heat.
Worn or defective machine parts (burrs or sharp surfaces on thread handling or stitch forming devices).
Machines out of adjustment.
Improper feeding.
Improper operator handling.