All EM games need to know when the score reel position is at zero. In order to successfully complete the start up sequence, all of the score reels are returned to zero at start up. If all score reels are not returned to zero then the score motor will continue to run, and the start up / reset sequence will not complete.
You need to ensure that the all zero position switches are open for the start up sequence to complete.

Here's a simple but clever 'Williams Score Reel' animation someone has put together, I found it on the web many years ago.

WilliamsReel.gif

Could be that it is a mechanical problem then. The problem reels may need to be pulled down, cleaned and checked. The backward latch behind the reel which prevents the score reel from turning backwards may need attention. Also check the EOS switch contacts are clean and gapped correctly.

Once a reel hits "zero" by virtue of the two switches being open, the reel stops moving, even if others still have not made it to zero. One of those switches is not opening or the wiring between the switch on the reel and somewhere else on the game is shorted. Solder splash, etc.

My suggestion is to tighten the two screws on the reel switch stacks. First tighten the one closest to the contact points, the the one closest to the wires.

Then check the adjustment.

This is one of the most common EM problems there is.

Photos per RBerlett's PM request:

Image 6904 (top photo) is the wire from top row, 5th to the left wire.
Image 6905 (bottom photo) is the bottom row, 4th to the left wire.

Both wires go the the large switch stack on the Replay/Credit Unit.
Both ID'd wires are the ones with the alligator clip attached.

IMAG6904.jpg

IMAG6905.jpg