Pixhawk 4 - motor does not spin

Hello everyone!
I am building an octocopter (tarot t18) and I am using pixhawk 4 as a flight controller. first time I connected it and made the motor test everything was good until I decided to fly it; one of the motors did not get signal from the pixhawk. I updated the firmware and it worked. after couple weeks I had the same problem again with different motor. After updating the firmware it worked again. what do you think the problem could be ? am I doing something wrong ?