ESC Calibration Not Working?

Hello guys I’m making a custom quad for a thesis but i’ve encountered a problem in esc calibration, I am using Pixhawk fmuv2 with custom Copter 4.4.4 and Fky Sky IA-6B- Fly Sky FS I6 combo. I’ve calibrated gyros , radios and all that but i’m having problems with esc calibration, I cannot calibrate the esc’s in PPM mode manually esc by esc, and I haven’t been able to use the mission planner to do it, when i follow the steps the esc’s just keep on single beeping(again ppm mode in rc) any help would be appreciated