You say that you have tried 3 or 4 times to come from oral sex, but haven't succeeded. First of all, I wonder if each of your attempts to come from oral sex followed an orgasm from intercourse. If this is true, then, you may not be waiting long each between'attempts.' Every man has a refractory period (sort of like the time it takes to reload a firearm). The refractory period can range from 10 minutes, to an hour, to several hours, to days, depending upon health factors and age. So, if it is true that you aren't waiting long enough between rounds, you could try oral sex first (instead of intercourse) or wait much longer after intercourse before trying oral sex.
If you say to me, but, I have waited and, no matter how hard she tries, I can't come. Beware of falling into the trap of pressuring yourself to have an orgasm. If you do, your girlfriend may develop lockjaw, and /or this issue will become your bone (or boner) of contention. Seriously, pressure and anxiety is the kiss of death to sexual arousal and orgasm. Relaxation and letting go is necessary to trigger orgasm. If your girlfriend is becoming bent out over your inability to ejaculate, you will feel even more pressured, so, make sure she understands that she, too, must not pressure you.After you both relax and eliminate the pressure, then, we can begin to learn how to have an orgasm through oral sex.
Once we are sure that pressure and anxiety isn't interfering with your arousal process, then, we can study what other factors might be interfering.
A common culprit is lack of adequate stimulation, which can be due to not receiving enough friction during oral sex. Many men learn to ejaculate while they masturbate and they have a hard time (no pun intended) adjusting to the softer touch of the mouth. If this is the problem, manual stimulation may need to be added to your sexual repertoire. Here's how this is done: You girlfriend should stimulate you with her hand, or any other way that brings you close to orgasm before beginning oral sex. Gradually, the time spent arousing you through other means can be reduced as you train your body to adapt to oral stimulation.
Also, learn to enjoy the sensations of nondemanding touching and arousal. Enjoying nondemanding pleasure is especially important because it helps eliminate pressure, which can prevent orgasm. It also teaches you how to increase your level of arousal prior to attempting oral sex. When your level of arousal is higher (either through a harder touch or a longer period of nondemanding touch), the chances are greater that you will experience an orgasm.
So far, I've given you two causes of inadequate stimulation--worry, and a touch that is too soft. Inadequate stimulation can also be caused by a lack of focus. In other words, if you pay too much attention to your partner's reactions, or if you get caught in spectactorism, (watching yourself rather than being immersed in the experience) you can easily lose track of your own sensations, never reaching the high level of arousal required for orgasm. Lack of focus and spectatorism can occur even though your are aroused enough to obtain and maintain an erection.
Now that we have completed our bedroom detective work, it's your job to relax, experiment and have fun. Let me know how you make out!