Anemone cannot dig into the sand unless it attach to something. Think of it similar to trying to dig into the sand with only one hand while you are diving. As you push you hand into the sand, your body float up. You must use 2 hand to dig under water. One hand to hold on to something so your body won’t float up and you push into the sand.
The anemone MUST attach to something solid before dig into the sand. I would get a SMOOTH CONCAVE rock and put it across the body and foot of the anemone. Healthy LTA will attach to the rock and then dig into the sand in short order. Once it reach the bottom of the tank, it can move around along the bottom of the tank. If you can see the bottom of your tank (some of us can looking up from the bottom of the tank) you can really see this. It is kinda neat.