gman
You are correct, you do have a problem and the dice probably are not staying on axis.
The problem is either that you are tossing the dice when they are splitting at the bottom while they are still in your hand (usually squeezing too hard or poor grip is the culprit), or your dice are wobbling while in flight which might be thumb placement.
Start first by gripping the dice using a mirror and looking at them while gripping. Secondly make certain that you are using the lightest grip pressure possible. If these don’t fix the problem then experiment with different thumb positions.
Be aware that all of these problems are very common. Good Luck and good shooting.
BTK

One way to check if your grip is too tight is to pick the dice up horizontal to the table, and just let them drop. If they are loose, they should drop close together. If they land far apart, your grip is too tight. Keep dropping them until your grip is loose, and keep practicing it. Too tight a grip is a very common problem.

Goddess

Also if you are opening your hand up at release it will cause a wobble the dice must roll off our finger tips.

A knuckle ball in baseball is created by pushing the ball with the end of your finger tips this takes away from the rotation. so when you open your hand the dice have a tendency to be pushed by the thumb also causing a wobble or taking away from the rotation.

Now as it has been said, light even pressure on the dice all the way around allowing them to pull out of your hand is the only way to go to gain any form of control.

Not allowing the dice to pull out of your hand creating proper rotation in a backward motion is like hitting a wedge shot with no back spin it will not sit down I bounces all over the place.

Even if you set the dice but they bounce all over the place it is not a controlled shot, remember you should be able to pick the dice up within a 8 to twelve inch area after they hit the wall