Power "headroom" is what keeps a hex or octo in the air after a motor failure. Being what they are, meaning not a quad, allows the loss of a motor and still sustain controlled flight possible, but only when the remaining motors and props have adequate thrust to compensate for the loss. The flight controller just tells each motor what it needs to do to keep things in balance but if those motors were near or at max output with all of them working they can't put out more than they already were to pick up the slack. To do it right requires each motor needs to have 3 times more thrust than necessary when everything is working right to provide a safe reserve. Something really heavy might need even greater reserves.
