Chedder hit the place you want to look.

R31 / C3 is your RC time constant and determines your reset period. The larger the capacitor value - the longer the recharge time.
Make sure the replacement C3 was 47uF and not 4.7uF. Also make sure it isn't an old NOS capacitor that could already be dried out resulting in a reduced cap value.

Also - if diode CR1 is shorted, you not get a valid result pulse -- will instantly end reset cycle.

CR1 prevents the IC input from exceeding the VCC threshold (+5V). The only time you will see this happen is during power down where the +5V line will go low faster than C3's voltage decay.
This will cause U1A input to follow the power down voltage rather than hold the U1A input high until C3 discharges through U1A and R31.