Me and my bf had this problem for a while and it was because Im a lot shorter than him. If there’s a big height difference between you 2, you might need to sort of arch your back more to make sure it goes it properly and to make sure you get the right angle in insertion if that makes sense. It’s something that takes practice and you and your partner need to experiment to see what works and what doesn’t. It could also be to do with if there is too much or not enough lube?
I’d also say just make sure you feel totally comfortable and not tense as when me and my bf did doggy the first few times we had this sort of problem as I was tense
