Sometimes when this happens you can't fix it, most of the time I find it is the road or wind. Sometimes what I do is twist the bars, don't turn them, that also works for me to turn while in a wheelie. By twisting I mean trying to torque the bike just a little so it moves the rear tire a little. Push down on one side and pull up on the other just a little bit will help. Also what you can try is bending your knees a little, the more you bend one knee then the other transfers more weight to either side. I very rarely have a wheelie just go perfectly straight, I almost always have to shift weight one way or another. Try that stuff out and see if it works