I don't usually have that problem w/my cat. But, there have been times that I had to clean back there because there was something stuck, or maybe kitty just didn't so such a good job.
When my cat gets wet (from a bath or an occasional squirt from a water bottle), he usually licks the wet area. Perhaps you could wet the cat's bum and see if she will then clean back there. If you do that every so often, she might get the idea.
How old was she when you got her? If she wasn't w/her mom long enough, she might not have learned proper bathing habits so you could try to teach her.
But, cats have their own personalities so it's possible she just doesn't groom all that much. The other times I've heard of this happening is when a cat is overweight; they can't reach back there too well. But, I wouldn't think that would be the problem of a kitty