My sebae now about 5 months old. When I first got it, it did the same thing. Even after I put it in a place it wouldn't float out of or get carried away by the current, it took a month to attach. Also didn't attach in the sand. I spent a week trying to bury the foot in sand (as suggested by someone) and he kept floating away and going upside down. Finally he floated into the rocks. Stayed there awhile, then finally attached in between some rocks and has stayed there ever since. Was a stressfull couple weeks watching him at the beginning.
Good luck!