I would check these things in this order:
Make sure the prop isn't slipping on the shaft by holding the gears and (not too forcefully) pushing laterally on the prop.
Manually spin the suspect motor by the prop and compare it's "feel" to the others (similar cogging effect or not and/or any slipping).
Barring any useful indications at this point open the shell, turn on the quad and run with throttle at about one third power, and
gently load
down each motor one at a time (by pinching, but not holding, the pinion gear on the motor) to "get a feel" for the difference between the "bad"
motor and the other good ones. Next swap the leads
at the circuit board from suspect motor with any other motor and test again running and
loading down the motors to see if the problem has changed location.
If it HAS NOT, and a known good motor now exhibits the same symptoms in that same location, you'll know it's a problem with the driver section
on the circuit board for that motor's location.
If it HAS changed location then you know it's a problem with that motor and/or wiring that was just replaced. Double check the motor to be sure
it's the same as the others and check the soldering connections at the motor.
IF you determine it's a fault with the PCB then be sure to
keep the bad one for parts (mark or remove the bad components) and just replace the
whole board (about $10). That way the next time something like this happens (fried driver) you can make repairs to your board.
These toy grade quads have everything on one PCB in them (unlike hobby grades where there are different boards for each function).
What would normally be thought of as the ESCs are just single MOSFETS and their accompanying diodes. Here are some pics identifying them
for you.
https://postimg.org/image/c5rwq2k7f/
https://postimg.org/image/774ebkizf/