In Bill Taylor's book, "Fitting and Drilling a Bowling Ball", he talks of changing the finger pitches laterally to get the hand to come around the ball more, creating more tilt and moving the track off of the thumb. This works if the track is barely clipping the hole, not more than a quarter of an inch.
If you are hitting dead center of the thumb you might want to try getting your thumb out of the ball during the release as it crosses your non-sliding leg in the down swing and turning the ball with your fingers. This also should pull your track away from the thumb. You will have to learn to adjust how much though, if you throw a spin biscuit then that would be too much, unless the lanes are torched.
