Answer: My first guess is that there is a lack of sexual arousal. When not aroused, your clitoris can feel numb or too sensitive. You may be going through the motions but your body is not ready or willing.
The next thing to consider is whether there has been a change in your level of sexual desire. If you are not experiencing sexual desire, it is much harder to become aroused, or be in the mood for sex. The hormone testosterone affects your level of desire and sensitivity to sexual stimulation.
Stress and relationship problems could affect both your ability to become aroused and level of desire. Stress from work and school can impair your sexual function, and long hours can leave you chronically tired or fatigued.
Please see the section about Desire, and in there there is a link to a page about Sexual Dissatisfaction that you should read too. Many of the biological and environmental factors that influence a woman's ability to experience orgasm are addressed on the page Information for Pre-orgasmic Women.
If you still have questions please feel free to write again.