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.

Fun fact: If your desktop background is black (like mine is) you can almost see the "+" icon. If you squint you can see its drop-shadow. It was there all along, but I couldn't see it.

Nice work, Apple.

(I'm on Yosemite, if that matters.)

the + button appears at top left if you switch your dock to the right.