As stated above - vaginal orgasm is a lot more difficult than clitoral so you are more likely to cum from clitoral stimulation than penetration. Generally you may find that you need to have a clitoral orgasm before you can have a vaginal one.

Time and patience are the keys here. Your partners need to spend the time exploring you and taking the time to listen to your feedback about what feels good and what does not. It can be silly lying there saying 'left a bit, right a bit...' but it is an essential step and once they get it, you will find it a lot easier. You know where you are sensitive and where you are not. He does not. To paraphrase Geoff in Coupling - 'when you are gay you have a whole practise kit to play with. When a man is with a woman, its live and you have to be able to fly that baby solo on the first try'

You also need to relax - both of you. I read a piece of advice from a tantric guide which applies to all sex which is that you have to have the attitude that the outcome is not important. If you focus on an orgasm being the outcome then you will end up getting stressed by it and risk it not happening (and beleive me, you may not think you are stressed but in all liklihood you are). If you remove this outcome based focus and just have fun you may find that you get there a lot easier. Try an intimate massage - some oil, some candles and a lot of gentle stroking on the body and genitals.

Also, have you tried using vibes in play? What about different positions? Or other sex toys? Have fun with it and see what works and what doesn't - just forget about 'trying for an orgasm' because that is the easiest way not to have one and go for it.