Like mentioned; take out the bobbin case, unscrew the drip pan, throat plate and go over all parts once again. Sometimes a thread or piece of a broken needle can get stuck in the feed dog - race - hook area. Do the same for the parts behind the face plate just to make sure. Wiggle the hand wheel gently, have a good look at the meshing gears and see if you can detect anything too. A good spot light on the work area is a must.