hi. this is very common. only about a
third of women are able to acheive orgasm
through penetrative sex. so there is
nothing 'wrong' with your girlfriend as
such, it just doesnt happen for a lot of
girls. this is largely due to the fact
most women need clitoral stimulation to be
able to orgasm, and because the clitoris
is an inch or so above the vagina,
penetrative sex (even though it may feel
fantastic) just doesnt hit the spot! i do
think there is still hope if you guys work
at it.
there is a position called coital
alignment technique which is basically a
variation of missionary. its difficult to
explain, but basically you are on top, but
rather than pushing the penis in and out
whatever way you usually would, you have
to ensure your pelvis is always in contact
with her clitoris. to do this you may find
it easier to have your legs out straight
behind (instead of curled around her) you
so that there is more pressure applied to
that area. when you are in this position
you both rock your hips rather simply
going 'in and out'. any time i've done
this i've found my clitoris is stimulated
with his pubic bone and it feels really
good. with a bit of practice, maybe you
can get her orgasming this way.
also you could try experimenting with sex
toys in bed. buy a small clitoral
vibrator, or egg vibrator that she can use
on herself (or you can use on her) while
you are penetrating her. or maybe a
vibrating penis ring?
good luck.