Turkish Horn bow

I have made a Turkish Horn Bow - bamboo, horn and sinew. Without a string on the tips line up perfectly. As soon as the string is on the bow twists and at the handle the string lines up with the outside of the bow. I have heated the limbs and tried twisting them. I have done this 4 times with very little movement. Do you have any suggestions? Do I just keep heating it until it moves? I don't want to get it too hot and effect the hide glue. What causes the twist?