my black and white clowns took a good while to host....close to a month.
I would also recommend moving the clam to a new location. you may have luck just moving the clam every couple days for a couple weeks. They will find the nem eventually!
My clowns were tank raised and I doubt they had ever seen an anemone before! it was really funny! at first they didn't even seem to take notice, but after a week or two they were obviously drawn to it, but they were very very cautious. I was actually watching when they first swam in. The larger of the two pecked at one of the bubble tips on the nem once or twice, then bumped it a few more times. After about 30 minutes of that game, the larger one swam directly through the tentacles real slowly once. Then BAM! dove right in! and when the smaller one was the bigger one snuggling it like crazy, it followed a minute or two later. Now they rarely stray more than 6 or 8 inches from it unless I'm feeding the tank.
I think clownfish snuggling on an anemone is one of the most fantastic things you can have in a reef! My Maroon clown in my bedroom tank is the most fun to watch! It really looks to be in ecstasy when it's in the nem! you can almost hear her saying "oooooOOOOOoooOOoHHH snuggle, snuggle, snuggle, snuggle snuggel, snuggle, snuggle"