On most of my songs the Spacebar toggles between Play and Stop. But on some it will only Play, it does not stop the song when you hit it a second time. I cannot work out why this is happening. Anyone got any ideas?

This is Reason 8.3 on Mac Pro software only and with a Novation Launchkey 49 plugged into the USB. (P.S. The Play and Stop buttons on the Launchkey work as expected.)