Virtualbox guest can't see shared folders

The host is Xubuntu 14.04 (up to date). The guest is Xubuntu 16.04. Guest additions are installed on both machines. My username is jjj (both machines) and I need to share ~/jjj on the host to the guest OS. On the guest the share appears under Devices > Shared folders, but I can't see the files on the host from the guest. I should add that under Devices > USB Devices on the guest I can see flash drives on the host and mount them on the guest. From what I have read this seems to be proof that Guest additions are installed. I should also add that the host has two Windows virtual machines, and ~/jjj on the host appears and can be accessed in both of them. It's just the 16.04 guest that can't see the share.

I've spent hours reading instructions here and from Oracle and elsewhere, and so far no luck. I need more clues!