My boyfriend was really difficult to get "it" in during certain positions...?

I agree with the first answer. Some positions make your body do different things..for example, doggy style might pull tighter down there and make you less open (if that makes sense, I'm trying not to say anything too gross) or it could be the angle or something like that. You can tell him it's because he must be too big, that will make him happy haha. Some things work for some people and not for others.