LTAs do not attach to rock long-term, they dig into the sand or gravel. So maybe that's why it never stayed before when you put it on the rock. The situation you've provided now is more appropriate for that species. Hopefully it will dig into the gravel you've provided and attach. Ultimately, if it does dig in, then it would be good to add more sand/gravel as a minimum of 4 inches is needed to keep them happy. Although nitrates of 10 isn't terrible, I would do water changes and try to get that to undetectable. Though well established anemones will tolerate nitrate of that level, it is an irritant that could make it more unlikely that your anemone will acclimate.