When I press Win+W expo only shows the windows of the current workspace. I remember 12.04 had a similar problem (it was a Compiz bug) and there was a PPA to solve that (see this question). The problem is that PPA does not seem to be maintained any more for Quantal 12.10 at least.
I really don't know if this is related to that bug or is just some configuration problem...
So how can I change this behaviour and get expo to show windows from all workspaces?