My girlfriend and I have been having sex for a few months. Were finding she doesn't feel anything when she cums. Sometime she does and sometimes she doesn't either way isn't very good. She describes it as building up as if she's going to cum and then nothing happens and her desire fades away. This has been a bit frustrating for us. The mood is always great, everthing is great up until the end when she experiences this. Any idea on what's going on?
There could be many reasons for what you described. Women are not like men, in that most women can only come under certain circumstances (although this is not to say that all women are dictated by the same factor).
One major issue many women have is environment, if they do not feel like they are in a safe environment, or if they feel uncomfortable for any reason some women can not come, no matter what stimuli is introduced or how skilled a partner. Although from your description I do not think this is the problem, you should definitely talk and make sure it is not a contributing factor.
Another common issue many women have while having sex, is they can only achieve sexual bliss in certain positions or while being introduced to external stimuli. Every woman is different, but most women say (in my experience) that the best positions for achieving sexual bliss is when they are riding their partner (This way they are in control and can do whatever feels best) or when they are on their back with their legs up (This allows for a much deeper penetration and tends to make it easier to hit the G-spot). Most women cannot reach this bliss if they are not face to face with their partner, this is because most women need to look into their partners eyes to feel the intimacy and security necessary for them to achieve sexual bliss.
Now there are a few women who cannot achieve sexual bliss during actual intercourse, my sister is one of these women, and there is actually a surprisingly high percentage of women who fall under this category. If you’re girlfriend is one of these women, which sounds like this might be the case, than there are alternative means for her to achieve sexual bliss. Some of the most effective ways involve various ways of stimulating the clitoris, from rubbing it, to licking it, to using a vibrator, and I have even heard of a few women who use electrical stimulation (although I would not recommending using this method unless all else fail).
All women are different, and the only way to figure out what works best for her is through trial and error. Sadly there are a few women who cannot naturally achieve sexual bliss, although it is extremely rare. Hopefully this is helpful and you can discover a way to help your girlfriend achieve that elusive sexual bliss.