I wish my thumb would wrap more around the neck. Everyone has their issues with their hands haha. Only advice I'd give is to keep practicing and working at it. What does the problem seem to be when you try to hold your thumb at the back of the neck when barring? Is it painful, awkward, or something else?

The more you move your thumb up and over the neck, the less mobility you get with your fingers in certain positions. There are many times where it is ok, and sometimes helpful to have your thumb wrapped over the neck, but for barre chords, I would think it only makes it more difficult. Just plant that thumb on the middle of the neck (backside) and barre with your first finger until you can get each string to sound clearly. Then add one finger at a time for whatever chord you are playing and see which ones give you the problems. If you are new at it and it's difficult, it's only because your hand needs to get used to the stretches involved and the new positions.

Oh, and there is always a "proper" or "by the book" way to do things on guitar, but in the end you'll see many people playing different. It all falls under what is most comfortable and easy for you.