On some assemblies when I create mirrored parts, say for an opposite hand assembly in the same file, I can adjust the distance of the plane from the face and the mirrored parts move accordingly. 

In other assemblies this does not happen and i am forced to delete the mirrored parts, adjust the plane to the new desired distance and then mirror again from the beginning. 

How to I make sure that the parts will always move to the proper location when i change the distance of the mirroring plane?