Good question. I've only had my FP since September but it is so tough to get that top lock that I consider it just dumb luck when it happens to fall in - sometimes up the right side; sometimes a new ball shot, sometimes bouncing off the upper right popper... and every once in a long while up the left side.
-Rob
-visit http://www.kahr.us to get my daughterboard that helps fix WPC pinball resets