She has to be completely relaxed. TBH it sounds like she isn't, from the way you are describing your relationship (shy/nervous), and it may sort itself out eventually as she gets more comfortable around you.
Don't give up though, I don't buy this "some women can't orgasm" - you do read this statement every now and then, but how could anyone actually have proven that? It's impossible to prove.
There's some anti-stress tea you can try, not joking, had very odd side-effect for me. (Tastes awful and no guarantee though, obviously.)
Get going with the all day revision thread