What was the level of sexual frequency before it declined?
Was there an event that took place (i.e. birth of a child) that could have changed things?
Has your appearance changed (i.e. gained a lot of weight) in a way that might make her less attracted to you?
Have you guys been to counseling?
If she's always been this way (with all partners), she could be asexual (www.asexuality.org). It might be worth checking out. They have a "for sexual partners, friends and allies" forum that might be of interest to you.
Many women don't like to be pursued sexually. Well, not after they are married anyway. The more she is aware of your lust for her, the less turned on she will be. It's largely subconscious, of course, but they have this feeling that the only reason you want her is because she's there. She doesn't feel like there's anything in particular about her that makes her desirable to you because you are always (to her) horny no matter what else might be going on. This makes her resentful and distrusting of you.
This is especially true if she has a negative self-image. She cannot understand how you can find her attractive when she doesn't find herself attractive. This diminishes your judgment -- and, by extension, you -- in her eyes. Once she doesn't respect you, she cannot feel sexual toward you.
The only way to get through to those types of women is to become more independent. The less you make her your whole world and start living for yourself, the more she will feel the need to reconnect with you, draw you back in, and keep you there. It activates her most primal survival mechanisms. That is the most potent sexual catalyst for her.
This has worked for me and for other men on this board.