I am able to calibrate each of the esc's individually by connecting them individually to reciever itself directly.
But when I try to calibrate all four at once its not happening.
I am doing the exact process for calibration as mentioned everywhere, like keep throttle up and connect the battery and so ...!
But I unable to make them to calibration mode.

I have done everything perfectly.
Right from the assembly to the calibration.
I have uploaded firmware Arducoptor v3.2.1 through misson planner and done every calibration work correctly.
But still when it comes esc calibration part its not getting done.
I am using Fs ct6b as transmitter and,
APM v2.8 as my flight controller.

I had no clue what might be the reason.
I have searched everywhere for the solution and haven't found any way, finally lost my patience and landed here hoping to find some help...!

I am giving two link in which is the connection and another is calibration
https://m.youtube.com/watch?feature=youtu.be&v=au0UJruUp4E

https://m.youtube.com/watch?v=z2G1P9bWM9w&feature=youtu.be

Calibarating the esc's individually will do the job.