Probably better for the ladies to answer, but I know on occasion, it won't happen to my wife. In fact, there have even been times when it didn't happen to me. Stress, pre-occupation with something else, illness, or probably ten thousand other things can cause things not to go as hoped for (by either of you). I'd say unless this is a regular thing, don't worry too much about it ... worrying could make it worse. If it is regular and the two of you can't pinpoint the problem, maybe she'd want to discuss with her doctor to make sure there's nothing else to be considered.

Just my thoughts. Good luck.

And, welcome to Pixies. Hope you enjoy your time here. It's really a great place!