stich bunchs up on the bottom?

seems after loading many bobbins with different thread. problem still there. If one pulls on the thread after it gets on the table one can pull for 6" or so then it's tight. pull it through the tight spot then it maybe Ok for a short time or it may pull erradic . We think it's ether the way the bobbin gets loaded or the bobin case . we have adjusted the screw, that makes no diff.