can't finish!!!! thoughts??

the last 3 times my boyfriend and I have had sex I haven't finished, it's not that he doesn't care he really tries and doesn't like when I don't finish. he doesn't last very long but that was never a problem before because he could always make me finish in a couple of minutes. But for whatever reason lately it just isn't happening!! It's like I'll get really close and then it goes away. Any thoughts? And it's definitely not that I'm not really into it because I really am! It's just so frustrating.