You also need add the extra groups to deform. If you add a magnet to the chest group it will deform only that group and look very bad. You will need to add r/l collars and maybe abbs or neck. depending on the amount of morphing you do. Magnet properties tab.

There is also the "edit falloff graph" in the magzone properties tab. This controls the magzone influence. If you want a nice smooth round influence ot a square /cut off type of influence.