It happens... The Host anemone species of percula are Heteractis crispa, Heteractis magnifica and Stichodactyla gigantea.
I'm guessing that you have an Entacmaea quadricolor? The fish might never associate with it, because it's not a natural host for it.
From my experience, wild caught fishes are more prone to associate. Do you know if yours are tank bred or wild? Also, some say that the size of the fish has something to do with it, but I don't know about that.