Sometimes when a KK2 is on a battery for a while, armed and idle, the synch is temporarily lost. I see this when I land and take off again in the last quarter or so of battery life 3.8 -3.7v.
Ensure you test for simultaneous start up on a fresh battery and freshly connected to the FCB, not after it has flown.
If a fresh battery does not turn the trick, pull the rotors and re-synch the ESCs to the KK2.
If that does not turn the trick. Rotate your motors. Put motor 1 on the boom for motor 2 etc...Does the problem follow a motor or is the problem stuck to a boom?
If the problem moves with the motors, pull (gently) on the connectors and see if one is loose or has a cold solder connection. If not then pull the motor that is sluggish and clean it out and try again or try a spare motor.
If the problem sticks with the boom, calibrate each ESC individually to the remote (not the KK2).
SimonK makes a HUGE difference. I cannot stress how much simpler it is to build and fly a copter running Simon K and light fast motors and 8" rotors.