Cleaning and lubing the Wiper Boards (Super Lube multi purpose synthetic grease)
is a big plus. Especially the Score Motor's and Flash Motor's Wiper Boards..

And how about on a single player, if you bowl 2 gutter balls in a row in the 1st frame,
does the machine score no points, but yet go to the 2nd frame?

Okay. If you bowl a Strike in the 1st Frame, and in the 2nd Frame
you get a total of 5 pins in the 2 shots, does the Score Control Relay
(on the back door) energize?
If not, check the N.C. (normally closed) contacts (along with its solder
joints) in the 10th Frame Control Relay, and N.C. contacts in the #1
1-10 Relay. Because the issue is with all players, very most likely not
in the Strike/Spare Step Up Units, but it could be in the Player Step Up
on Plate 1, and I think that's the inside plate (but can't remember 100%).
I'd first check the contacts (and solder joints, wiring, ect) in the 2 Relays
mentions, and if the Score Control Relay still doesn't energize with a strike
up, then and open (which should count the amount of pins made twice)
then I'd inspect the wire on the Player Step Up Unit's Wiper Board plates..

The only score that's correct is #1.
If a Strike is made in the 1st frame, an X will show.
Then if an open is shot in frame 2, (we'll keep it 5
pins total between the 2 shots), with a Strike up,
the 5 pins should be counted twice for a total of 20
going into the 3rd Frame, and no mark showing..
On your last post, 29 is accurate after the 3rd frame.
The open in the 2nd Frame should count those 5 pins
twice when a strike is up. It's the Strike/Spare Step Up
Unit being stepped up 2 steps showing a strike is what
tells the Score Control Relay to energize which breaks
the circuit to the Relay Bank Reset Coil.
Then on the Score Motor's 2nd revolution, the Strike/Spare
Unit is stepped down (reset), the Score Control Relay
doesn't energize, and the Bank resets for the next frame..