Imagine you’re the flight controller for a minute. One of your main tasks is to keep the drone level, and on demand you’re supposed to pitch, roll, yaw, climb, fall… now you get the command to climb (somebody slightly increased throttle on the transmitter). What do you do? You increase motor speed, but nothing happens (because there are no propellers attached, but you don’t know that). So you increase motor speed more, still nothing happens. You also notice that your vehicle is not completely level (because the desk it stands on has a slight tilt) and you try to compensate by increasing the speed of two motors even more, but to no avail.
That’s what’s happening to your flight controller, more or less. As @amilcarlucas said, use Motor Test on the bench. You need your transmitter only for flying.