check your upkicker switch, it may be gummed into the on position making the machine think there is a ball there, then you dropping a ball in there loosens the switch enough to cycle it normally.

The VUK uses an opto, but yes, that too could probably use a cleaning.

The usual problem with an old Bally saucer hole switch is that the switch nub is worn down so that the switch doesn't work to kick the ball out of the hole.

Remove the ball from the game and go into switch test and see if the saucer switch is reported as closed.

In your case someone may have over adjusted the switch to compensate for the worn nub making the switch always closed, or the contacts are gapped too close. To get the switch to work consistently correctly you may have to add some length to the worn switch nub. I have used heat shrink tubing around the nub adding about 1/16" and filling the head space with dab of silicone.

This is the Gottlieb SNS for those that are responding. Go into switch test and see if the VUK opto is on. Check all other switches while in there and report back. Gottlieb Sys 3 optos are notorious for getting cold solder joints. Easy fix though.

Aha, a Gottlieb system 3. If the Vertical Upkicker switch looks and checks OK, it maybe that the game is looking for a lost ball. Remove all balls and check ball switches for correct closures in switch test. Use balls where possible to make switch closure.