If he cannot revive her, there are a few things I suspect.
One is because of the protection of whatever Gods they worship, her soul, apart from her remains, cannot be affected by Necromancy because of their protection or blessing.
The second which I think is more likely is, that her soul has already moved on. Dwight can no longer use Necromancy to call for her soul, because she's already back as someone else.
A lot can happen in the time it took for Dwight to become a Lich.