Fore-skin doesn't roll back?

Am 21, uncircumcized and the foreskin of my penis doesn't get back like in most penis' uncircumcized at the time of an erection. It just stays like it is while erect or flacid. I saw a doc sometime back, and he said it's a rare thing in some people but very normal. Anybody else know anything bout this? I know almost all circumcized men so cant really inquire there.