Nature...
Soley nature and time.
Sometimes wild clowns host quicker, sometimes not.
My OC's were afraid of an anemone when I got my first one. The Cinnamon clowns jumped right in. When I moved/upgraded everything to a new 180, they still stayed away from it.
When I got a new anemone, they jumped in it before it had even been 15 minutes in the tank. Made the Cinnamon's mad... but.. finders keepers...
Tank age needs to be at least 6+ months and stable with no swings to host an anemone. They are like an allergy sufferer during hay fever season. All it takes is the rock to off gas some junk that you do not see or can test for and they get ALL MAD!!!