If you notice that your stroke is crooked on your follow thru, a technique I use will show you your stroke problems. Shoot a bunch of long straight in shots, but stand about 5-6 inches further back from the shot than your normally would. Make it so your bridge hand is very far from the cueball, roughly so that when you hit the cueball it will be at the end of your follow thru. When your arm is at a 90 angle from the floor, your tip should be 5-6 inches away from the cueball. Try to focus on hitting center ball. If your stroke is crooked you wont be hitting center ball. To fix your stroke I recommend the third eye stroke trainer, which is probably the best tool to fix your stroke in the world. This technique of bridging really far back along with using the third eye will give you a dead straight stroke. If your problem is the backstroke, then Doug Carter's stroke trainer can fix that too.