Something similar is happening to me with gnome-shell-3.10.1-2.fc20.x86_64 in Fedora 20. Sometimes however it is possible to drag windows to workspaces that have other windows on them already but I can't seem to ever drag a window to the last empty workspace.
So it looks like a gnome bug to me.
Regards.--
Mads Villadsen <maxx krakoa dk>