Dropbox Sync has deleted files from DL4100?

Hi -
I’ve got a WD DL4100 and have a dropbox business account.
Over the weekend I set up a sync through the dropbox app on WD to sync a couple of folders from DL4100 to dropbox account.
Back to work Monday morning and a couple of the folders have synched across and are still visible in both DL4100 and Dropbox, but one folder has not synched and has disappeared from my DL4100.
I’ve looked in the recycle bin, but nothing on either DL4100 or in dropbox.
The folder has just disappeared.
Any ideas???