See what you have on Pins 15 & 1 of Z21.
Also see what you have on Pin 4 of Z36. (It should be the same as Z21 Pin 12.
Z36 doesn't look to be the issue, as it has a resistor R48 tied to +5 as a PULLUP resistor.
This means there is +5 volts on the (INPUT) Pin of Z36 unless something is making it LOW.
Larry.