I can not drag more than two text objects into Incopy assignments from Indesign

I have an issue with handling multiple assignment elements.

I can not drag more than one or two - not three or more - text objects into Incopy assignments from Indesign in one of my multipage work documents. Editing all stories assignments does not work either. It produces idml-files, but they are not shown in assignments.
Quite annoying, if I have to drag two and two objects at a time. Other docs work well, so its a problem in one document. Does anybody have an idea what I can do to solve it?