Brushless systems run by powering the 3 wires that go to the motor at various times. Depending on the location of the Rotor inside the motor, different different ESC phases (A,B,C) or different motor sensors are in use. It is possible for 1 phase or sensor, to be damaged, and the motor still sort of run or work, but not really work correctly.
This can show up as a lack of overall power, or the vehicle needing a "push" to start sometimes, or not smooth operation.
The following information is to help you trouble shoot your system for this type of damage on the bench.
This can be done installed or removed, but you'll need to be able to see the pinion shaft and pinion gear.
Think of the face of the motor as the face of an analog clock.
With 12, 3, 6 and 9 Top, Right Middle Bottom and Left Middle.
Rotate the Flat of the motor shaft (Or the pinion gear set screw) to point at each "Hour" of the clock face and try to start the motor slowly by applying throttle.
If there is phase or sensor damage, you will find one section that the motor will "not start" but if you move it passed that, it seems to operate.
Depending on the level of the damage, the motor may run roughly or smoothly or slowly or seem almost normal.
The next step to trouble shooting this is to confirm if it is the motor sensor, or the ESC that has suffered the damage. Testing the motor, against a 2nd ESC, or the ESC, against a 2nd motor is the most direct bench top testing.