Why can't I cum?

I've been sleeping with this guy for a couple of months now and no matter what - time, place, position, etc - I just can't get off!
He gets off every time without a doubt, but I really just can't come and it's starting to really infuriate the both of us. I've been with other men before who have made me cum so I know what it's like to orgasm and I'm just not reaching that climax :( 
Is it him? Or me? 
Help???