I use Oracle's VirtualBox for my virtual machine needs. For me specifically, I still use a very old copy of Outlook running in Windows XP and clipboard sharing is important. Unfortunately, I had been having problems with copy and paste failing for the past half-year. I.e., I can't copy from the host and paste the content into programs in the virtual machine. And I can't copy from the client virtual machine and paste the content into programs in the host. If I reboot the Virtual machine, copy and pasting would work for a little while and then stopped.
Well, at least for me, the solution is to update the VirtualBox Guest Additions program. With your virtual machine running, select Devices -> Insert Guest Additions CD Image, and then install the Additions via the CD drive of your virtual machine.
I had been neglecting to update the Guest Additions in the client virtual machine while I update the VirtualBox program. Seems like eventually the version incompatibility creeps in and breaks copy and paste even though other services supported by the Additions still works fine.