Sometimes encounter no ESC pwm signal output when flight control is started

ArduCopter Copter 3.6
1

Hi,I have a problem. Sometimes encounter no ESC pwm signal output when flight control is started.I watch the signal with an oscilloscope. Restarting a dozen times may appear once.
Firmware: Copter3.6.9

2

What flight controller are you using

4

@smartdave Use Pixhwak1.Build Firmware Select FMUV3！