If you don't have a physical reason for not reaching orgasm, then the thing for you to focus on is not, "Why can't I come," but "How can I have an orgasm?"
Here are a few ideas for you to try:
- Make sure you're good and turned on before you even think about having an orgasm. Try thinking of a fantasy, read some erotica or watch an erotic video to get you in the mood.
- Experiment with different positions and techniques. Oral sex may work great for one woman, but others find penetration a sure way to orgasm.
- Masturbate. Can you bring yourself to orgasm? If so, teach those techniques to your partner. Or better yet, masturbate with your partner as a witness.
- Try a vibrator. Many women find that the best way for them to consistently reach orgasm is through the intense stimulation of a vibrator. Don't be afraid to bring toys into the bedroom if they increase your sexual pleasure.
Sources: The Whole Lesbian Sex Book by Felice Newman and The Survivor's Guide to Sex by Staci Haines