If you're feeling adventurous and you've checked to make sure all the usual suspects (ie friction or worn linkages or dirty parts/contacts) aren't your problem, and you'd still like more powerful slingshots, you can remove layers of wire from the coils themselves. Remove the coil, unwrap the paper, unsolder one end of the wire from a lug and remove wire (which makes the coil more powerful - to a point) until you're happy with the amount of "sling".
Just be careful not to take off too much wire - you reach a point of diminishing returns eventually. I've had good luck with that on pop bumpers on some early games.
Like someone else noted - the yellow and orange dot coils are for flippers - 3 lugs.