I’m not a Marvelous designer user but it sounds to me your having more then one part in some type of group, either find way to select sub-elements within this group or find way to attach all elements withing the group so you get one complete object.
No idea if this info helps you in any way (probably incorrect) but it’s my best guess.
You probably gonna have more luck asking these types of question at (https://www.marvelousdesigner.com/community/forum)