I have had a pair of clowns in my biocube for 6 months now, My Hammer and Frog spawn are hosting one, It took about 4 months for him to do this. The female is just starting to go into the hammer. I do have BTA that they do not touch. I say just let them be, When they are ready they will or will not be hosted by a coral or nem. Just takes patience as with anything in this hobby.
My 90 has 2 Sebae clowns that will not leave their BTA. Also I just let them do their thing and the Nem host them. Took 2 weeks for the Nem to host.