I would watch the behavior of your clowns. If they are biting around the rocks the base of the nem is attached to, this cleaning of the surface (which is a pre-spawning behavior but even if they are not sexually mature they sometimes engage in these types of behaviors as sort of a ritual/practice) may irritate the nem so it moves. One of my carpets has been in the same exact spot on the same rock for nearly a year. When the clowns spawn, they bite at the foot and the disk to keep the carpet off of the clutch. Depending where they decide to spawn, they sometimes get the anemone to completely change shape. Over the course of a day or so of this, the anemone moves 2-3 inches to the other side of the rock to get away from the nipping.
A new anemone would likely be even more apt to move if the clowns are engaging in pre-spawning behavior.