This may be a stupid question, but do you have an empty house the fulfills the requirements for an NPC?
The three requirements for the nurse are that she needs an empty house(obviously), need health increased with a life crystal, and the merchant needs to be present. If these three are met, my only question as to a potential conflict would be if your character was full health *before* making this world(as in, you "imported" it over). I don't know if this is a possibility, but if you are already full health and can't use a life crystal, maybe her spawning criteria couldn't be met? I'd try using a life crystal on a new character on the world and see if that works, if it is indeed the case. But this is all speculative.