One or two of my motors don't spin as fast as the others do. What should I do to fix this?

Lorena -

This is normal behavior when flying in ''Stabilize'', ''Loiter'' or other flight modes that are assisting you in leveling the vehicle. But, under a controlled environment, when testing or arming the motors for example, this should not be the case. If this is happening to you, you should Re-Calibrate your ESCs. Please check this video for instructions.