Well there are a few things you can try.but a few questions to start off. First, can you shoot a square shot to the big dot(release the ball at the center of the goal and have it go straight into the goal, not being condescending, but I don't know what table you are playing on), or does it spray? Secondly, are you trying to go too long? thrid, are you following through on the forward part of the stroke back toward the opposite wall to help square it off?
It would really help if I saw your entire set-up, ball positioning, stance and stroke, but I'll see if I can talk through it with you, one step at a time.