Assign to Desktop x option missing

I've noticed that my Mac no longer opens applications I've specified on my second screen. When trying to set the option again via right clicking on the icon and going to Options, the "Assign to Desktop x" is no longer displayed.

What could be the cause of this? There is definitely two screens attached to my Mac as they both display different desktops.

(I am using Mavericks)

Solution 1:

Following this discussion on Apple.com :

You'll have to delete the following preference files :

• ~/Library/Preferences/com.apple.spaces.plist (if you have multiple space.plist delete each of them.
• ~/Library/Preferences/dock.plist