The thumb has three joints just like your other fingers, the first joint is at the wrist. Sometimes the base thumb joint gets pushed back. One thing I do when this happens. Slightly moisten the wrist above the joint. Then, with the other hand, gently grip the top of the forearm with your finger tips line up the heel of the hand a bit above the joint on the wrist area. Then, abruptly, with some force snap your hand into the base thumb joint. If it is out of place, it will jump forward.
If you are having trouble with the middle joint, you have to get the preasure of the bow off of the thumb to some degree. If you go to a straight gripped longbow and place the bow so that the thumb takes most of the load, it will make your condition worse. I make certain that I get the bow deep into my hand. With some recurve grips, I have the same problem that you have, which one of the reasons why I prefer longbows.