By default, gnome 3 only uses workspaces on your primary display. This is fine if you only use your laptop screen (with no external monitors connected). When you connect external monitors though, the secondary display is treated as a 'fixed' screen. Workspace switching, by default, only applies to the primary display.
To enable workspaces to switch for all displays:
gsettings set org.gnome.shell.overrides workspaces-only-on-primary false
By default, gnome 3 only uses workspaces on your primary display. This is fine if you only use your laptop screen (with no external monitors connected). When you connect external monitors though, the secondary display is treated as a 'fixed' screen. Workspace switching, by default, only applies to the primary display.
To enable workspaces to switch for all displays:
source: http://gregcor.com/2011/05/07/fix-dual-monitors-in-gnome-3-aka-my-workspaces-are-broken/