Have you checked the height of the feed dogs relative to the needle plate when the needle bar is all the way up? With the needle plate securely attached and eye level with the needle plate, hand cycle the machine and see if the dogs are coming up enough. I had a machine which would only make short stitches and this was the problem. There should be a single screw holding the feed dogs, if it's adjustable as I think it is, don't make them too high or they can hit the needle plate, test by adjusting then cycling the machine with the needle plate in place but not screwed down and see if the dogs lift the plate when they come up.