Magnepan Ultra Tonearm

I just purchased a turntable with magnepan tonearm there seems to be some unwanted movement in this piece. when you try to set the weight the tonearm becomes less responsive by grasping the cartridge shell and pushing back towards the counter weight the arm will move back and forth I am quite sure this is not normal but my knowledge of this tonearm is very limited. If anyone can offer any insight it would be greatly appreciated.
Thanks
Wayne