Triggers dying

[Mint_Sauce]

Imagine you have a ball that rolls towards a door, when the ball touches the door the door opens towards the ball sending it away again. The whole thing is on a slope so the ball then rolls towareds the door again and the whole thing repeats.

My problem is, when the ball has touched the door several times it just stops working. :x

I can throw a grenade at the door and it will trigger, sending the ball away again but the ball won't trigger it, ever :hmph:

Any ideas why the trigger just stops working?