Here is what has worked for me:
Find a place in your tank with medium to good light ("good" defined as SPS level), medium variable flow, and a concave section of rock. Dig down, next to the rock, into the sand a couple of inches (I assume you have 3" or more sand). Place the LTA with its foot in this depression. If it is healthy, your tank provides a suitable environment, and you chose well, the LTA should settle down and work to attach its foot to the bottom glass of your tank. If you have provided a good environment, it should stay put for years. Mine hasn't moved in 17 years!
What size sand are you using? How deep is the sand bed? Consider posting a picture of the anemone for some of the experts here to look at. Maybe it is not well.