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.