Provided the feed dogs are engaged (worth checking) and moving correctly, the most likely cause is that the needle has a barb or hook on the point preventing the machine feeding the fabric through. You need to change your needle regularly (every 6-8 hours) and ensure that they are fully inserted with the 'flat to the back' (most modern machines).
Another cause can be the needle breaking and there are snags on the needle plate or presser foot.
Have you cleaned under your needleplate? It is possible the feed dogs are so clogged with lint and unable to move.