Unlike many other tools, magic can and will select objects and cells deep in a hierarchy. But if you select a cell instance that is further down the hierarchy than the immediate child of the edit cell, then you won't be able to move it because you're not editing the cell definition that the instance is in.
You may need to select multiple times until you have selected a cell that is an immediate child of the edit cell, or you may need to load or edit a cell down in the hierarchy (you can use the menu Options->Cell Manager for this).