Stress is a BIG reason for many women's low sex drives. Try getting her relaxed and being loving and romantic to her. It may work and it may not.
Worst case scenario would be that she just is not a very sexual person. If that is the case, just way whether the status quo is sufficient to stay in the relationship.
One question though, is she just slow to start and then gets into it and has orgasms, or does she just seem to be tolerating the sex just for you?