I have problems with my drone based on pixhawk 2.4.8 (here is a full set)
After arming the drone, some of the propellers have trouble with starting to spin here is a video, when I increase throttle, most likely they catch up here you can see it, and everything is fine.
- MOT_SPIN_ARM is set high enough to spin the motors cleanly.
- MOT_SPIN_MIN is set high enough to spin the motors win a minimal level of thrust.
Do a motor test and see how high your startup throttle needs to be.
Check ESC calibration, connectors and the above parameter settings
Go to the tuning wiki if you have not.