What about if her hormone levels are low? Test, estro etc. Maybe that is the problem?
What if it is? What is SHE doing to get to the bottom of all this? Do you see where we are all coming from?
ETA: We are all looking for 'initiative' on her part (CeMar calls it desire). Constant initiative. Not something you have to push her into, but from a place she finds within herself. SHE has to want it... THEN you can work with her.