Anemones normally close up, turn into snot while dying. If it's not doing this, try gently placing it in a location. Is the flow too high. They do not like high water flow.
I would decrease your water flow for a day or two to let him attach, A 2 liter bottle with holes cut in it placed around him would do the job, allow him to attach to the rock and then remove the bottle.
I would decrease your water flow for a day or two to let him attach, A 2 liter bottle with holes cut in it placed around him would do the job, allow him to attach to the rock and then remove the bottle.