Does the motor stop if you remove the ball, or send it through the trough to the shooter lane? It could be that the outhole kicker isn't working so the motor keeps trying to kick it out.

/Mark

Does the DB/Reset Completed relay ever trip?

I'd start with that then:
4 Square DB relay (resized).jpg
One of those six switches may be dirty or not closing.

The Runout switches are on the Score Reels. They close when the Score Reel shows a zero. As each Score Reel reaches zero during reset its Runout switch closes. Once all four are closed the DB relay should fire.

Given that the game was working I'd limit yourself to carefully cleaning switches for now rather than adjusting anything.

That's a picture of one of the (10, 100, 1000) Point relays. And yes, most of those switches should be open.

What you're looking for are the switches on the Score Reel (in red):
Gottlieb Score Reel (resized).jpg
The wire colors on the schematic (from reply #6) will tell you which of those switches is the Runout switch on each Score Reel.

As far as we know the DB relay is not the culprit. It's more likely the victim of the circuit that drives it. One or more of the switches in its circuit (from reply #6) isn't closing.

Essentially the DB/Reset relay is waiting to hear back from the Score Reels that they're all at zero. Even though they are at zero, unless the Runout switches are all electrically closed at zero the DB relay won't know they're at zero.

So the DB/Reset Completed relay still isn't firing reliably? Have another look at reply #6. There are 6 switches that need to close for the DB relay to fire. Two of them aren't on the score reels. One is on the XB/Game Over relay on the trip relay bank and the other is at 2B on the Score Motor:
Score Motor 2B (resized).jpg
The Score Motor is often mounted to a large hinge with a pin holding it in place. If you remove the pin the motor should swing up for better access to the switches. Just be sure to replace the pin when you're done.

The trip relay bank also offers better access. It has wing nuts on either end. If you loosen the wing nuts you can swing the entire relay bank up for better access to the switches. It can be a little fiddly to seat the bank back into the proper spot when you're done but take your time and it should wiggle back to where it belongs.

If the problem is that the DB relay doesn't fire then I don't think it's a switch under the playfield.

If getting at the Score Motor or Control Bank switches is daunting consider using a jumper to bypass the suspected switch(es). HowardR often recommends that approach, for example at: https://pinside.com/pinball/forum/topic/capt-fantastic-bonus-hesitation#post-6240269