So far I have had 7 clownfish of 3 different species, maroon, ocellaris, and percula. I have never had one not host one of my btas. One pair took about a year, but eventaully they did host. I have heard from very seasoned vetrans (10 yrs +) experience that all clowns will entaully host, it is natural behavior. In the ocean once clownfish go through metamorphsis, they must find an unoccupied anemone to host or die. It is not learned, it is instinctive. I would advise you to be patient.
It has been documented that if the anemone is not the clowns natural host, it takes them a little longer to buld up resistance to the anemones sting but other than that they will usually accept a substitute eventaully.