A ball that tracks over the thumb hole is usually the result of the release. If there is little hand rotation or wrist **** throught the release, the ball will tend to track high(on or near the thumb). To try to get the hand rotation without much effort, keep your pinky right next to your ring finger and spread your index finger away from your middle finger some and see if you naturally move the track.
If that doesn't work, see where your hand is during your swing, and where it is at the release point in relation to the ball. A semi-roller is accomplished by keeping the hand behind, or slightly inside the ball going into the release and turning the hand slightly toward the outside of the back of the ball at the release, which should have the thumb coming out of the ball at or slightly before the sliding ankle.
If you can visualize a clock face under the ball with 12 o'clock pointing toward the pins and 6 o'clock back toward the settee, you can use these for a righty. The hand should start with the thumb in the 12 o'clock position and move toward the 10-11 o'clock position at release with the fingers starting at 6 o'clock and ending up in the 4-5 o'clock position.
Some people try to keep their hand in one position through the entire swing, but if you can rotate your hand without over-rotating(spinning the ball) you can be more effective in changing your roll and getting more power in the back end.
Don't forget about the other basics, though. Keep your shoulder relaxed, along with your thumb, while keeping your wrist and fingers firm. If you have trouble feeling where your hand is, have a friend watch, or get a video camera
. Hope this helps.