Possibly a dirty switch or spider contact that is limiting current to the knocker during gameplay? Unfortunately I don't have a schematic. Is the knocker volume different for a match as compared to a score replay?

If you manually depress the plunger on the add replay (i.e. credit) unit solenoid with the game on and the knocker fires loudly, then that eliminates the knocker coil itself, the switch on the add replay unit, and likely a normally closed (NC) switch on the extra chute relay.

It's possible that the add replay unit solenoid is being weakly energized due to dirty switches that are limiting the current. Do you have a schematic? If so, start with the match circuit by looking to trace the horizontal circuit that goes from the right side vertical power buss through the 1) ?? motor switch (might be motor 2C?), 2) a normally open (NO) switch on the match relay, 3) the plug on the match in/out adjust in the lightbox, 4) the copper traces and spider contact on the ones score reel, 5) the traces and contact on the 0-9 unit, and 6) the 15th position switch on the replay (credit) unit (closed if less than 15 games on the credit reel). Clean and adjust all switches.

If the knocker then sounds loudly for a match, would then have to adjust/clean the switches on the circuits from the 100 point score reel that also fire the add replay unit solenoid.

PS, I was going by the schematic from a 1965 Gottlieb K&Q. Your circuits could be different.

Maybe start by cleaning the 15th position switch on the credit unit and the plug on the match in/out adjustment. Those would be the easiest to access and should only take a couple minutes.

I was out of town and missed your last question. Were you able to figure out which switch, wire, or contact resulted in the weak knocker action?