There are many reasons that a woman cannot reach orgasm. These can arise from a variety of causes. The main ones being:

Inadequate stimulation:

A majority of women believe that climax should come from intercourse alone or from short clitoral stimulation. For some this may be the case but the majority need more stimulation over a longer period of time to achieve climax.

Performance anxiety:

You may find yourself so desperate to reach climax to be sexually adequate for your partner which causes stress. Your body needs to be relaxed and you must not be pressured into performing.
This can become a vicious circle leading to loss of sexual libido and taking away the pleasure of intercourse.

Medication:

Many types of prescription drugs can stop you from achieving climax, these include - anti depressants, birth control pills, anti hypertensives. Consult your doctor for more information if you are concerned.

Bad sexual experience:

You may have been abused or badly hurt in the past which can lead to mistrust of men, a lack of confidence in yourself and loss of sexual contact with others.

Bad relationship:

Being in a bad relationship can cause resentment, mistrust, violence all causes instability which again unsettles a relaxed environment needed to be open and receptive to sex.

Emotions:

If you feel ashamed or embarrassed and ill at ease this can undermine sexual arousal from any kind of stimulation.

These are only a few examples of common problems for not reaching climax. Try to talk to your partner and talk openly about any issues you have, take things slowly and most importantly enjoy yourself.
Consult your doctor if you are on medication and see if the drug you are taking is affecting your sexual response.