You can assume that one rotor turning too slow because of a faulty motor or ESC; however, you should check to see that you have the wires for your motors connected in the correct place and polarity. You can check this by how your rotors turn. Blades should spin faster in the direction of your rudder input (right=bottom/left=top). Check to see that nothing is binding with the bottom rotors (i.e shaft bent, gear rubbing aginst something, etc.). That should help with that problem. Hope everything works out.