In my experience one of the key-factors for a woman to experience orgasm is that she feels comfortable.
Unless she is completely comfortable in your arms it will be harder for her to orgasm.
If you have her over again, take your time, perhaps start with a relaxing massage, no pressure. Do the whole Kama-Sutra ritual with baths, massage, kisses and relaxing environment, scents, etc.
What is also needed is a tension, a sexual tension to be specific.
This may be triggered by different factors in women.
Some need to feel dominated, some I have experienced have only been able to orgasm by getting oral.
If she's hardly ever experienced orgasm, it may be a good idea to start by giving her a clitoral orgasm, before even attempting to let her have a vaginal one.
David Shade has a brilliant method for clitoral stimulation, but you need a fair squirt of lube. (it involves direct stimulation of the unhooded clit, which is very, very sensitive, thus the lube.)
It's called the welcome method. You'll find it in David Shade's Manual.
btw. I doubt she's a squirter, or at least, I doubt that is the problem.
But if you want to find out, you probably know how to give a girl a g-spot orgasm.... just don't go there straight away, take time to build with clitoral, and deeper vaginal stimulation.
I presume she is able to come on her own ?
Most girls I know come much easier if they can play with themselves at the same time as I'm fucking them.... With more than one girl the best position has been what I've heard called the orgasm-position, or side-by side position. (where she lies on her side, with her back to you, her shoulders tilted a bit so you can kiss, keep eye contact, or such. and you are on your side, with her closest leg over your thighs.. It's kinda hard to explain, but it leaves her legs spread and unhindered access to her pussy from the front, so it's easier for either of you to finger her.)
But anyway; start out with clitoral orgasm: It may help to stimulate her nipples too, (produces oxytocin). and then move on to vaginal (this is easiest to achieve with your fingers deep in her, stimulating the deep spot (look it up, anterior fornix) ) and then move on to such as g-spot, anal, etc.
Hope it helps.