Your bta is on the glass near the surface of the water? What kind of lights do you have?

An lta that won't attach when
1. not happy with the water movement<usually this is the reason
2. its annoyed with other things in your tank. examples : inverts, other anemones, fish or parameters.
3. its foot is damaged

Long tentacle anemones like calm waters and they like their foot in the sand. How deep is your sand? How long have you had the long tentacle?