Cannot Arm Quadcopter (APM 2.8) with Spektrum DX6e

Hello

I can arm my QuadCopter (APM 2.8) from Mission Planner. I recently received Spektrum DX6e and was able to bind it successfully to its receiver. tried to arm motors by holding the throttle down, and rudder right for 5 seconds. Nothing happens. When i arm it using Mission Planner, it arms well, and when i use throttle I can hear the motor spinner harder.
This means that ARM is receiving a signal from Spectrum transmitter. But i do not understand why the RC transmitter cannot ARM the quadcopter.

Many Thanks

Ammar

Calibrate your radio from Mission Planner like described here: https://ardupilot.org/copter/docs/common-radio-control-calibration.html

And make sure you have no trims on the tx