Did she automatically end up in the dungeon, or did you *send* her there, because she might fight against being sent to the dungeon.
Also, if that's not the problem, she might have an addiction that gradually saps her health, or you might want to fire up the WMEditor and check the girlsx file to see if her health's not just set low to begin with. I've had a couple girls, who because their stats weren't set correctly to begin with ended up arriving in my dungeon dead.