I think because the original rubber sits so high on the post, the ball might often wedge itself under the rubber, instead of rebound off it.

Once the targets are dropped out the way, i guess the ball is hitting the rubber at higher speeds and getting stuck a lot.....my theory 

The lower rubber is the 'added' one, and is relying on the top one keeping it in place.

Anyhow, i guess when i've cleaned up the game and re-rubbering it, i'll soon know if i need 1 or 2 rubbers after a few trial games.

MM.