If your girlfriend is not reaching orgasm, it is nobody’s fault—there is simply a lack of awareness due to preoccupation with a climax, compulsive thoughts or even performance in general. In some cases, a woman’s vagina has become numb or calloused due to overstimulation or past sexual trauma, which could also be a contributing factor.
Do not despair, our natural state is one of perpetual orgasm—it is simply a question of being present enough to recognize that which is always here. The bliss of enlightenment (orgasm) is always present within you, but cannot be recognized by an active mind. Relax and empty yourself of past and future.
Also, you leaked two key words which reveal doership and effort. You said “No matter what I do” and “I can’t make my girlfriend orgasm.” Your word choice suggests that you not only think you’re doing it, which is an illusion, but also that you’re forcing it. Love is effortless. The more relaxed you are, the more loving you are, the more orgasmic you are.
You apparently have a lot riding on whether or not she orgasms—perhaps your self-image as a “good lover” is at stake? There is no such thing as a good lover, only compatibility or one who loves themself. This love generates the energy of sexual ecstasy, so attend only to overflowing with love and everything else will flow as well.