Lets assume there are 3 electromagnets in a brushless motor. They are oriented in a way that if any 2 are on you get the rated torque of the motor and it does what its supposed to do. If only 1 is on it has half the rated power. You need 2 of them to be on to accelerate the motor from 0 but 1 on will keep the motor rotating if its already at some speed.

a bad motor, wiring, sensor, esc are all possible culprits. when this comes up I try to work in this order:

1-remove load from the motor (what happened)
2-check all of my wiring with a multimeter
3-take a b or c off one at a time and when it does the same thing you dont like that is the phase with the problem.
4-quick winding check-disconnect from esc and short all 3 phases of the motor to the case (paper clip or something). Rotate the motor. In between the bumps in the rotation you will feel a fluid resistance in the form of rotational drag and it increases the faster you go. Make sure its symmetrical between each one of the bumps.