Rebuild the stepper Mark, chances are that its either dirt and grime that adds extra friction and prevents the spring from doing its full work and so it wont fully go to "reset"position.

On some games also. its common to have the centre nut act as a tension adjuster... Stoner used this method and it can be bloody irritating as its really easy to over tension. Once stepper is cleaned and free from grease and dirt just make sure you tension the nut whilst checking the steppers action to  full and reset position.
Putting more tension on the spring wont solve a problem it just delays it for a bit and youll be back later.... but hang on a minute... this is Kidamingo yes? Im teaching you to suck eggs!

If main solenoid still does not pull in then follow its earth path by digging through those wire looms and determine what relay activates it. I often have to do this with the woodrails and by time schematic arrives from PBR ive fixed the machine anyway.

Good luck mate..

Gav