the motor you put in may be the same size, but if it has less turns then it wont have the torque to spin the prop fast enough.
or, the motor you put in has many more turns than the stock motor, so it doesnt spin fast enough.
either way, to many or to few turns and it wont spin the prop fast enough.
when it comes to anything with a prop, the motor specs have to work with the type of prop. props with higher pitch require greater torque to push the prop through the air.