I had used a apm2.8 flight controller to build a f550 hexacopter .I had done all the configurations and all motors spin when I connect usb cable to laptop and then when I am arm separately in feild its not spin.
It refuses to arm because something is wrong.
The simplest solution is to replace the outdated APM 2.8 with a newer FC and newer firmware (Arducopter 4.4.1).
The newer firmware will tell you in more detail what is wrong.
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