I once had such a problem with gloves rigging. In that case I had deleted the neck actor cause not neccessary. The rigging was BODY-hip-abdomen-chest- and then left and right arm parts. So normally the neck is obsolete cause chest is the common parent for the arms.

But with using a morph the gloves left their positions. After reimplementing the (empty) neck actor this behavior was gone and all worked correct. Might be a symmetry problem ...

May be it's the same with your parts ??