If the water flow has not been changed/reduced in any way & no new additions, it might be time to look at bulb replacement. These are generally the only reasons an anemone that has been long term stationary will move.
weird.. mine moves every 5-6 months as well...very weird.. he is on the right side of the tank and slowly drifts towards the pump uptake... its like it wants to die.. i've recently moved it back and replaced bulbs all looks fine..