If you're practicing in an open field, try to get the disc to release where the body is allowing it to happen...basically don't try to hit the line exactly how you think you should. Set your body up and see where the discs are going. See if there is a trend that the good shots are all coming out 10 degrees right, then see if your stance is too straight rather than too closed. If you set up too straight and are always trying to hit the straight line, you'll early release relative to your stance every time because you are thinking about where you are trying to throw. If you just let your mechanics take over, then evaluate after if there is a "griplock" trend on better throws...then see if it's actually a negative griplock that you need to fix, or if you are simply setting up incorrectly but your mechanics are doing ok. Think about if someone is trying to serve in tennis with a straight/parallel stance...they are set up for failure from the start, and a higher velocity serve will look way off line compared to what they are "trying" to do.