If an aircraft motor is failing to properly spin up it is usually due to either:

Potential sources Causes What to inspect for
Motor damage Major motor damage is usually caused by either a hard crash, or particulate build up in the motor itself. Inspect the motors on arming, if one motor stutters or stalls slightly before arming, this can be a sign that the motor is damaged or obstructed. Remove the motor from the aircraft using the provided 1.5mm driver, and unplug it from the system. Run this motor under water, allowing it to dry fully before re-installation. If this does not solve the issue, replace the motor with any spare.
ESC damage ESC, or the component that directs the motor how to spin, can be damaged due to a hard impact, or by attempting to operate a damaged or jammed motor. If replacing the motor does not solve the problem, it is likely damage has been caused to the ESC. If this is the case, the system will need to be sent in for repair.

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