Do they run if you help them? That is a known problem and the reason we have a kickstart function to give them more power when you start from 0. It's defined here
/** Some fans won't start for low values, but would run if started with higher power at the beginning.
This defines the full power duration before returning to set value. Time is in milliseconds */
#define FAN_KICKSTART_TIME 200
/** Defines the max. fan speed for M106 controlled fans. Normally 255 to use full range, but for
12V fans on 24V this might help preventing a defect. For all other fans there is a explicit maximum PWM value
you can set, so this is not used for other fans! */
#define MAX_FAN_PWM 255
But in your case it will kickstart with reduced MAX_FAN_PWM so that might not work if 63 is not enough.
Try adding
#define COOLER_PWM_SPEED 2
and see if that improves it. It reduces the on time in favour for more signals, but resulting in same average power. Might be that the fans like that more.
