Sometimes a capacitor that should be good, just won't take a charge, or does
so slowly. If a capacitor has been idle for some time, it can act this way.
Sometimes it needs a jump (like capacitor in parallel), a prime, or high
voltage to wake up, and then will function properly. The capacitor acts as
if it has some sort of internal corrosion that needs to be burned off. Do
not exceed the voltage rating of the capacitor when doing this or you may
permanently damage the capacitor. Also be sure to observe polarity when
applying a capacitor.
For example, the large capacitor (C23) on a Bally Voltage Regulator/Solenoid
Driver board was okay, but wouldn't pull the voltage up high enough.
Jumping with an identical capacitor in parallel at the board level didn't
work, but jumping at the bridge rectifier (BR2) did the trick.