If the voltages on the opamp are all correct (as per the voltage chart on the troubleshooting guide) then that part of the circuit is probably working fine. The only thing I can think of is either one of the resistors is not the correct value, or perhaps a solder joint is not making a good connection. If you can’t figure it out, you could take some (detailed, in focus) photos of both sides of your circuit board (clearly showing all the solder joints and components…you’ll have to bend up the pots on the solder side of the board to show under them) and I could see if there’s anything that I could notice that could be causing the is problem.
Good luck.
-Brach