For women, sex is very intertwined with the brain and emotion. If a woman is not feeling sexually attractive or has body issues or issues with sex in general, such as feeling guilty or that sex is "dirty" or a necessary evil that you have to endure, it can keep you from achieving orgasm. Being with a partner who you are 100% comfortable with and who is as concerned about your needs as they are their own, is also key.
When you are totally relaxed and have no anxiety about the act of sex and all that goes along with it, it can be a very satisfying experience. But even things like being exhausted from taking care of the house and kiddies all day can take away from the whole experience. When our minds are focused on other things and we are not "present" in the moment, it can be very difficult to reach orgasm.
Take time, alone if possible, to just explore and experiment. Try to make it a time when you absolutely know you will not be interrupted by anything. Also, the majority of women need clitoral stimulation to achieve orgasm. If you haven't concentrated on this are before, give it a try. You might be very pleasantly surprised! Best of luck!