I have a Windows host and an Ubuntu guest. I have drag-and-drop and copy-and-paste enabled both ways. When I try to drag or copy a folder over I get the folder but none of the contents. Tried with other folders to different locations. Same result. Kind of a noob when it comes to Linux and VirtualBox but after a while of looking I couldn't find anything. Guest Additions is installed.
This may help. https://forums.virtualbox.org/viewtopic.php?t=4386
Also do you have quest additions installed.