I found it out in a previous thread of mine.
The actors need to have a Persistant Location set up. It's the second tab in the reference window. If you don't set it, it defaults to the location of the cell they are in. This means that if you have no locations set up for your unique cells/worlds yet, then it defaults to nothing, and they cant fill. However, if you place an actor in vanilla Tamriel, it fills and works just fine.
Set up your locations and it will work, or until then, make them Specific ref's. Then once you get your Loc's set up, change them.
