Did you check to be sure the feet are lint free?
After sewing too many denim quilts, I had problems Cleaned everything (I thought), re-threaded and still nothing.
DUH- there was so much lint caught in the feed dogs that it was like pulling out FELT from between them. They were so bound up, they couldn't move, so nothing else could, either.
AHHH. Now it works smoothly.
Susan