I had my clowns with BTA's for over a year, never got them to associate. Put a H. Magnifica in the tank and the clowns were in it within 30 minutes.
Most people get false/true percs, and most people also get BTA's. That combination is not natural in the wild and can result in clowns that never associate with the nem. Like stated above other clowns will normally go straight into a BTA as it is their natural host.