Can't orgasm?

So I've been dating my boyfriend for 6 months now, and when we have sex I usually orgasm without any problems. But for the past few weeks, it's been really hard for me to have an orgasm.. My boyfriend makes me super horny and I always get in the mood, but I have trouble keeping the feeling down there (it usually comes on pretty intense and then it suddenly goes away and I don't come). Any suggestions as to why it's happening to me? And how I can start having orgasms again? It's getting very frustrating.