Maximum number of credits appears to be 1, but ROM and the DIP switches theoretically support max of 5, 8, 10 or 15. Not just 1. It would be good to understand why this is happening.

This is quite possibly related to the accumulator problem and possible RAM problem discussed in https://flipfix.theflip.museum/problem-reports/75/

http://www.pinrepair.com/sys1/
https://www.pinwiki.com/wiki/index.php/Gottlieb_System_1

Accumulators / bookkeeping do not seem to work. Specifically, even after tilts and games, the machine reports zero tilts and zero games.

My chief suspect at the moment is bad RAM

Replace the power supply’s C1 5/12 volt Filter Cap

“…Unfortunately C1 at 2900 mfd (or 4700 mfd) is way too small of a filter cap, and the fact that this cap is 30 years old isn’t helping things either. Replace this cap with a new 6800 mfd to 10,000 mfd 16 volt or higher filter capacitor.
When replacing this capacitor it is NOT necessary to take the whole power supply apart! Just cut of the old capacitor off the power supply board, leaving the old cap leads as long as possible…”

-from http://www.pinrepair.com/sys1/