I had my false percs for almost two years before the figured out to host my frogspawn. I tried all the suggestions. Feeding into the tentacles, pics of clowns near the FS, etc. Nothing.
Then one day I noticed that the smaller one always slept in the same lower corner of the tank. One day I put a single head of FS in that corner, and within two weeks the smaller clown was hosting that color of FS in my tank. After a nother couple of weeks, the other one joined it. After another couple of weeks, they were hosting almost any color of FS that is in there (I have probably 8 or 9 different color variations of FS and hammer, with a total of over 50 heads).
Don't give up on them. They just may take a little while.
dsoz