Problem

Only some digits are working on the display. Everything else works normally.

Solution

Verify the display itself is functional by pressing the TEST button, which will light all segments on the display.  If the digit is still out, the problem is with the display itself.

If the digit lights with the TEST button depressed but does not operate during the game,  there can be a problem with the cabling to the display, the connectors, or the MPU board itself.  Check all cables feeding the display by gently wiggling them. If the digits intermittently work, a connector rebuild is recommended for Molex^© wire-and-pin type connectors or replacement ribbon cables.   For ribbon cables, test with a known good cable if available. 

If the digits fail to light, attempt to use the display in a known good system.  If the display is verified as working and the cables are good, then the problem lies with the MPU board itself on the game, which should be professionally repaired.