Can't do doggy style

Mariah
So my boyfriend and I have tried and tried again to do it doggy style and we can't.. We use lube and I just don't understand. I've done it with other guys and it was no problem. I know I'm in the right position, and I have a big ass but I feel like that doesn't matter, and he's not huge but a little over average.. So what the deal? Any ideas???