I had that happen to a short sinew-backed mulberry bow I made once. While the sinew was drying on it, it twisted the tip on one side off alignment. Funny thing is that I had already left the tip quite thick just in case something like this happened. I think two things could have solved the problem now. 1: lay the sinew more carefully insuring that all the threads are totally straight. 2: keep the bow a bit longer or wider than you want so that you can shorten it or thin it if it decides to twist. Of course if it does a propeller twist, you are just screwed unless you leave the bow significantly thicker...which would eliminate the treated belly. In the future, I will be more careful when I select wood for a bow like this! My advice, just play it safe and use the best piece of wood you've got. Sinewing is a real time investment.