Girlfriend can't achieve a climax, why?

I've had with my gf about 25-30 times now. I've had with a couple other girls and they were both able to climax without too much work. Now my girlfriend seems to dry up much quicker than the other girls. I was thinking is it possible that she dries up before she can climax? She isn't bothered by not achieving an , but I feel bad b/c she deserves one. I've had with her for an hour and still got nothing, but no lubricant was used she just managed to stay wet. Any advice would greatly help we're both 22, and she has ed in the past just only a couple of times i guess.