If you couldn't give your girlfriend that big 'O' experience, first talk to her. Find out if she is enjoying sex with you. Know that some women may have difficulty in experiencing an orgasm even though they enjoy sex with their partners. Your girlfriend could be that type. She may not be able to tell you the exact reason for not experiencing an orgasm, as she may not know it though.
Now, you buy a vibrator and ask her to use it till she receives orgasm. From close quarters find out which kind of stimulation gives her great arousal and that which makes her experience orgasm. For some girls clitoral stimulation is important for achieving orgasm, while for some sufficient stroking of vagina is needed to hit the roof. For some, cuddling, teasing, dirty talking and sensual teasing, foreplay, oral sex and whole bunch of other factors are important for them to reach climax. You need to understand what makes her experience orgasm. You need to be patient, talk to her or understand through your observation. Just pushing your guns harder and longer may not always work out.
Watch what she does with a vibrator. Take a look at her movements, how and where she paces it, rhythm she follows, speed and where and how she strokes etc., to achieve her orgasm. You just repeat the same with your organ.
If required, try using a lube. She might not be wet enough down under. When you introduce a lube things might ease out and the new sensation that she experiences may make her to experience orgasm at a quick time.
Know what gives her a high, clitoral stimulation or g-spot stroking.
Try oral sex and change the pace and introduce variations to allow her to experience different sensations. If required you watch some tips of cunnilingus and know the art and science of it.
You can also begin to focus on some of the erroneous zones of her body like neck line, boobs, hips etc., While you may be too focused on pleasure hole down under, you might push her to orgasm even by just playing with her boobs, who knows? Some women react to different sensations differently and who knows which body part can trigger her to throw up.
So, you need to be little patient and spend some time exploring her needs closely. When you do that, there is always a way that you can make her receive an exploding orgasm. You might just take few more attempts, that all. But it is not something that needs mastering rocket science. It is paying attention and understanding her needs, as simple as that.
Last but not the least, try the time tested technique. Make good use of her ears; talk dirty or compliment her beauty and her flawless body. Even playing a good music can set the tone for an exhilarating orgasm.