I think it's because you and her did it so much that it's lost it's appeal to her. Some people secretly desire variety and that includes having a "new body" to have sex with. I know it sounds bad but it's true. It's like anything you overdo, you can get tired. She only does missionary because she's not into that much and being downright lazy. You can't force her to have sex with you. The only thing you can do is pray and hope for the best. Even if you talk to her and counsel with her until you are blue, if she's not interested or looking for something else, you can be wasting your breath.