The reason why it's not attaching is because it's not happy where it is. It wants to blow around and if behind the rocks is the place it stops, then that's where it's "happy".
If it's not ideal back there for it, it will creep up the rock on it's own.
You can't force an anemone to position itself in the ideal viewing spot. Where they end up is where they end up. This is why many people do without them. they can cruise around the tank and sting everything they touch until they are in the ideal place.
Don't worry, if it stays behind the rock, it will be viewable pretty soon, because they grow very fast.