Hard to provide a full answer without seeing you cast. Are you casting like Lefty Kreh or like Joan Wulff/Steve Rajeff? One problem I used to have was that at the end of my back cast the fly would bounce and I would lose line tension and the fly would go all over the place behind me. The solutions: execute a slower back cast or begin the forward cast earlier, a few feet before the back cast unrolls. If you're casting like Wulff and Rajeff try not to pull your elbow back during the back cast. The elbow does move back, but mostly because of reward body rotation. If you pull your elbow back you will lower the rod tip from the target line on both the back and forward casts, and, if you're casting a heavy fly, you might hit and break your rod tip. Personally, I don't cast heavy flies. Instead, I use sinking tips,

Randy