I've ran across this problem a few times and it happens to be a little problem that can become very frusterating:

I prefer to mate by selecting two to four faces at a time (using ctrl + left mouse button) and then selecting mate by using the hotkey (ctrl + spacebar) I have set up. This makes mating very quick but every once in a while SW will refuse to let me select more than one face at a time.

Any input as to why this may be happening would be appreciated; I would imagine it is a simple fix but not one I've been able to figure out!