Can't create a new desktop workspace? When I bring up the spaces option (by either pressing the F3 button or 4 finger swipe up on trackpad), the + button in the top right corner is no longer there and I can't create a new desktop space. 

I am running mavericks 10.9.4 on a 2012 13" MacBook Pro.

Any suggestions/ideas?
The + button only appears if you move your mouse close to the location of the button, or hold down ⌥. Also note that you can have a maximum of 17 desktops, or 16 desktops and Dashboard, then the + button is no longer available.