Matt, if I had to put money on it I’d guess that your luff rope has shrunk, which means you can’t get the head of the sail right to the top and so the back of the boom sags down. Good news: you can fix this yourself! Check that it is actually shrunk (the front edge of the sail, the luff, will be all crinkly). Then there should be some stitching at the bottom of the embedded luff rope which you can remove. Then stretch the sail with a second person, and re-secure the luff rope in its new position. Hope that helps!