What am I doing wrong?

So me and my boyfriend have sex A LOT. And by a lot I mean for hours every single time we see each other, which is almost every day. Every single time, he makes me orgasm at LEAST three times. When we first started having sex, everything was normal (but not as intense). Now that we've learned more about how to please each other, I feel as if I'm doing something wrong. For the past probably 4 months, he hasn't been "finishing" as he used to. We try for hours and hours, but still NOTHING. I try focusing on him, and he says it feels amazing, but he just can't cum. He says it feels like he's about to, but then it just goes away. He doesn't know why this is happening. I know it's not that he isn't attracted to me, because he gets hard just about 99% of the time he lays eyes on me. Since this started, it's not as if he hasn't been able to cum AT ALL, but it happens every few weeks it seems. Just recently it had been two or three days in a row, but I guess I jinxed it. So if anybody has any idea why this is happening, I'd really appreciate you sharing :)