There are so many factors that can be affecting your ability to reach orgasm. For example, if you smoke, use drugs (including some prescriptions), or drink excessively any of these factors can increase the amount of stimulation you need to reach orgasm. Also, if you are experiencing a great deal of stress, through work or otherwise, it may result in a lack of ability to focus on your pleasure during sex.  Another possibility is that you may be too focused on the mechanics of sex and reaching orgasm to truly enjoy it.   You can also try cutting down on the frequency of sex or masturbation to increase your sensitivity.  If you engage in sex too frequently, your body can become almost numb to the sensations.

If none of the above descriptions seem to fit you, there may really be nothing wrong at all. Society places a great deal of emphasis on the timing of orgasms, which can create an unwarranted feeling of inadequacy. There is a book I will suggest, which ironically matches your name - The Big O, by Lou Paget.  This book explains some tactics for heightened stimulation and points out that it is not quantity, but quality of the orgasm.  I suggest that you new goal should be to make each orgasm more magnificent.

-Zaida