as said above, I have read numerous sources that say juvenile clowns have to accostom themselves to the sting of a particular type of anemone...so if your clown is not used to this type of anemone then it will have to get itself used to the sting...contrary to popular opinion clowns DO get stung by the anemone, but their slime coat somehow deflects the stings and overtime they are immune to its effects.
I had a clown in a tank with a BTA for 6 months before it would host in the anemone...prior to that is hosted in a bubble coral...
also, sounds like the clown has been in the tank prior to the anemone, so it may be accustomed to the tank and where it likes to hang out...somehting that has worked for numerous people is to remove the clown for a day or so...rearange the rocks and put the clown back in...then it is much more likely to host the anemone quickly...its the same principle as when you add aggressive fish at the same time...their memories are short and after a day or two they forget their "territory"