Doggy Style

Hey y'all so this is kinda weird but whenever my fiance and I try to do doggy style I can't get a good arch? And his dick just slips out. 😂😂😂 Like it won't stay in. He's about 5 or 5 1/2 inches, if that makes a difference. Just wondering if anyone else had this problem and if there is any way to avoid it?