Material is moved forward by the presser foot pressing down and the feed dogs moving up to grip the fabric. Like two hands pulling together, the fabric advances. Proper feed requires the proper movement of the feed dogs in time with the needle and hook movement.
To identify proper timing: advance the needle until it is just about to enter the needle plate on its downward movement. Observe - the feed dogs should be level with or just below the needle plate. If the stitch length is at zero, the movement of the feed dogs will only be up and down. If the stitch length is at its longest setting, the feed dogs should ba at their farthest back position.
While the needle is under the needle plate, the feed dogs move from back to front under the needle plate. As the needle rises, the feed dogs rise and move to the rear.