APM won't arm

I had my quad completely assembled and calibrated, ready for first flight. Then decided to update to 8 channel radio and configure power for use with camera servo gimbal to be installed later. I connected the new radio pins 1 - 4 and 6 - 8 to the APM input pins. I pulled jumper pin 1 and used one BEC to power the output buss and another BEC to power the input buss. Radio calibration and flight mode setup all seem OK. Connecting power with USB cable or Power LiPo results in steady blue led GPS lock and blinking red led. All Flight Modes set to Stabilize for initial test. Radio calibration check shows all radio inputs moving in correct directions. But no matter what, cannot get APM to arm.

Frustrating since the system worked great with 6 channel radio and one BEC with JP1 jumper installed. System passed all tests and would arm. Motors would spin up when throttle was advanced. But I wanted to make it better with more capability and now can not get APM to arm. I reinstalled JP1 and pulled the second BEC, but still cannot get system to arm. I am running out of places to look. Any suggestions?

I found the solution - this is how: A YouTube tip suggested using the Flight Data display in Mission planner to show why the APM would not arm. When you try to arm a copter with a Pre-Arm Safety Check problem, an error message appears over the status line on the attitude indicator. I received a “Acro_Bal_Roll/Pit” message. So I went into the All Parameters screen and set everything to default values. Basically I had to start all over with calibrating my APM. I tried to skip the Compass calibration step and got an error message about the Compass when I reconnected my APM to Mission Planner. So I went back to the Initial Setup steps and ran the compass calibration routine. Low and behold, my copter now “arms”. Now I have to go back and set up my radio and Flight Modes again, but I have a much better understanding about how to troubleshoot this area. Hope this helps another pilot!