When the stack is collapsed Lightroom cannot "see" the other images. It knows that there is a stack, but all images are assumed to be virtual copies. A virtual copy cannot exist without its existing master file. I don't know if there's a solution from an engineering standpoint. But it is what it is. Sorry for the inconvenience.
Which version of Lightroom are you using?
All the selected photos are virtual copies. Only Master files can be moved to another folder. f I make one of the Master files the top one of the stack or if I expand the stack, it works fine
Any stack containing Virtual copies cannot be moved in Lightroom.Reproduced the same at my end and it works when stack is expanded.
Let me check that with our Engineering team.
the stack can have multiple virtual copies and will move with no problem as long as a virtual copy is not the top image. It may be that Lightroom can't see into the stack, but that is totally lame.
Agreed, it's a defect that would be easy to fix. You can move a collapsed stack with a virtual copy inside, and you can move an expanded stack with a virtual copy on top, but not a collapsed stack with a virtual copy on top.
Please file a bug report in the official Adobe feedback forum: Recently active topics in Photoshop Family about Photoshop Lightroom
Tip: To expand a stack and select all of its photos, shift-click on the stack count in the upper-left corner of the thumbnail.