Unable to have sex in any other position than missionary

Me (28M) and my girlfriend (27f) have a pretty nice routine that works for us. We do a lot of foreplay, then switch to oral, before switching to Intercourse. But for some reason, my penis is entering inside her only when we are in missionary. When she gets on top of me, she can't seem to enter it inside her. The same happened when we tried doggy style too. This has never happened to the both of us with other people.

Is there any reason for it to happen? She says that she's not stressed out and is highly turned on/wet when we've tried her on top of me but it just doesn't seem to work out. Any reason why we can have sex in missionary but not any other position?