When pushing pitch stick, one propeller is visibly much slower

It’s an X8 configuration drone.

When pushing the throttle stick forward, all props are turning at the same speed, from a perspective of the human eye.

But when pushing also the pitch stick forward in addition to the throttle, the lower back right prop visibly turns much slower than the rest of the props, which seem to be turning at normal high speed.

Also, after doing this throttle and pitch sequence and landing, the motor that turned slower is colder than the rest of the motors and the motor above that motor, is hotter than the rest of the motors, as if compensating for the motor beneath it.

What could be the cause for this? What is the way to find the cause? And what are the possible solutions?

If you’re doing this test on the ground without props or with the drone fixed to the ground, or tethered then forget it. The control loop is not able to react properly because the planet is in the way. Might be fun to do, but it’s a useless test.

Use the motor test in Mission Planner to make sure everything is spinning in order and the right direction.

If the drone was flying free, then post a .bin log of the flight in question. Might be able to see a discrepancy between the two motors.

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