Those sliding horseshoe contacts in 70s and 80s Williams System 6 (and System 4) drop targets - unfortunately they can be unreliable from getting dirty or out of adjustment. The targets don't always score, or the bank won't reset after the last target drops. It affects game enjoyment more than you would think!
Sure, you can take them apart and clean them...but they'll get dirty again sooner or later. In some machines it's a major hassle to disassemble & clean them because of other stuff in the way (e.g. adjacent pop-bumper assembly).
Wow, didn't mean to sound like a car salesman. But I've been at this for a while and there's a whole background and history for this thing at one of the other well-known pinball forums.
Anyway, a working replacement has been designed and is nearly ready for prime-time. Have a look at these videos:
Post edited by Steve_T : Long video replaced with a shorter one.