Just be patient. They do that, sometimes they come out, sometimes they dont. Try to keep an eye on it tho. If you can get a little mirror on a stick or something so you can see it back in the rocks. If it looks like its dying, get it out quick. Nothing will foul a tank faster than a dead anemone. The bio-crap it turns into when it dies can kill other livestock.
You may try re-aranging the flow pattern in the tank and see if he moves.
Anemones rarely survive in new tanks. Your tank should be a minimum of 8 months old. What kind of lighting are u running?
anemones will go where ever they please in your tank, its pretty impossible to convince them to stay right where you want them.