Can't do missionary position?

Okay this is about to be a little tmi and I'm sorry but I'm not sure what to do!😂 So I have the most amazing boyfriend ever who is amazing in bed, but we really struggle to find different positions to try. I suck at riding, and I really want to try missionary but it seems like every time we try it feels like nothing's in there? Like he'll thrust but it's almost like it isn't going in. We'll usually try for a few minutes but it doesn't work so we go back to what were doing. His size is definitely not the problem, so I can't figure out why it isn't working? If anybody else has had this same issue or has any tips pleeeease let me know!