http://www.greenfoot.org/scenarios/4074
I've made it happen less often with the last 2 updates, however it still sometimes unlocks from the balloon - can someone look over my source code and see if they can work it out?
Whenever any balloon gets to the top, it sets lock = null, even if it wasn't the balloon that was being locked onto.
So the lock is lost once any balloon hits the top of the screen.