I think what Brad is saying is that when the base (fleshy part) of his thumb hit his mouth he is locking up and not able to get the fleshy part of his thumb back any further to reach his anchor with his finger.
I think that is what he is saying because that is exactly what is happening to me right now. Ironically, I've somehow acquired this problem by working on gaining more "control" at anchor on targets.
I've always been a pull through guy. Hit anchor and away the arrow goes. My hand would move straight back an inch or two. That along with my instinctive shooting style has been great for hunting and stumping, but I was looking to improve on targets so I started holding at anchor.
Now for the cure... I don't have one! I'm still fine hunting and stumping, but it's a most difficult chore to get that last inch back on a target. I can do Jay Kidwell's drills all day long without a problem. I can draw and anchor perfectly if I know I'm not going to release. I can even trick myself by telling myself I'm not going to release, anchor perfectly, and then release, but there is no flow to that and the concentration is not on the target. Bow weight is not an issue. Same thing regardless of a 36# or 66# bow. Yeah, I know....Mental!
Don't mean to hijack the thread but if you guys have anything to add, I'm listening.