That glitch is usually caused by the mesh not being at origin. It is easily fixed in 3DS Max by toggling snaps, adding a sphere at origin, converting the sphere to an editable mesh, selecting the sphere and attaching the armor, selecting by element, deleting the sphere, importing the original armor, adding UVW mapping modifier to it, dragging the UVW modifier to the new sphere armor. Simple. In other cases, it's because your mesh is weighted to geometry instead of bones. Make sure you only add bones to you skin modifiers. I would post this in tutorials but I don't have the permissions necessary to do so.