It could be a couple of things. In order of likelihood:
* If you made copies of existing NPCs and didn't remove their old AI packages, those packages could be telling them to be somewhere else.
* You may have flagged them as initially disabled.
* They could be (enable) parented to an object that is disabling them.
* Your area may not be navmeshed.
* They may be falling through some areas that do not have collision into the void