Desktop numbers keep changing


I'm using MacOS 10.9 on a MacBook Pro 15-inch, Mid 2010.

When I hit Ctrl-UpArrow and move Desktop 2 to the Desktop 4 position, it looks like it takes. And for a little while I can hit Ctrl-4 and get to it.

However very shortly after (less than an hour), when I hit Ctrl-4 it takes me to the desktop that was originally 4. Hitting Ctrl-UpArrow shows the desktops have returned to their original positions.

Is there a way to make them stay?

Thanks in advance.