If they are tank bred clowns they probably have never seen an anenome in their lives so the chance of them hosting in one is remote. Even if they do know what to do with an anenome there is no guarentee as has been mentioned. I have had clowns take months to even show an interest.. some have jumped right in and others have never shown interst their entire lives.
They may also not be interested if they sense the anenome is under stress or sick. Is the tank ready for an anenome and is the lights strong enough?