Some women have difficulty orgasming...Or it could be improper stimulation
try this on your own...
Push 1 or 2 fingers up your vagina and feel around on the top for a wet, spongy feeling spot. Stimulate that area while rubbing your clit and see what happens.
Orgasms are also a mental thing. Do you want to cum? Are you turned on enough?
Try lotsa foreplay and get really aroused too!
(lol im a guy btw and I know tons about sex :p)