Why can't I orgasm?

Me and my boyfriend have quite a lot of sex and it's absolutely amazing ! Like I can feel myself leading to an orgasm but it never happens but it still feels great. We've done cowgirl doggy etc but still nothing and I get worried it makes him feel like I'm not enjoying it because I've told him I've orgasmed before masturbating myself which must put him down a lot. Is there any ways I can increase myself getting  one or is this a regular occurances with some of you ?