When I try to zoom behind the teeth the zoom is getting slower and slower until I can't zoom anymore, and I am still not near the back of the teeth. Someone know what this can cause?

This is just something that happens when you zoom (dolly) too much. You need to reset the dolly state by focusing on a selection. I recommend you tab to edit mode and select some verts in your mouth, then hit the period key on your number pad to focus on that selection. Then your zoom / dolly should be reset.