OP - I don't "finish" sometimes. Stress plays a big part. That and, my sex drive is honestly no match for hers. I kept up for a while, but eventually it gets difficult. She used to take it personally, but after we talked about it she understands. She told me that she thought I didn't find her attractive anymore, maybe I was cheating, etc, etc. Now she just goes with the "I got mine, to hell with you" rule.
Hehe
It's probably nothing to be worried about. But I'd strongly suggest you two talk about it. If you don't, you'll always wonder why. And personally, I always felt bad about it because I could see it bothered her.
Just my two pennies.