why does it not arm?

Everything is charged, plugged in, ready to go.
But nothing happens! It does not arm!

That can have a lot of different reasons.
Below is a checklist to inspire your troubleshooting:

- is your transmitter bound to the receiver in the model?
- do you know/did you confirm what arming method/channel is configured?
- do the stick/switch movement exceed the required min and max values?
- is the gyro and the accelerometer on your flight controller calibrated?
- does the FC sit level?
- do you know the stick commands for acc and gyro calibration?
- if you have an OSD, it might be possible to display the reason for not arming

The most common reason for not arming: the inputs/outputs on the transmitter are not configured properly.

If you have a flight controller with a USB interface, connect it to a computer and confirm that your sticks and switches do EXACTLY what you expect them to, and if its different, you could adjust validity, trigger ranges and dead band on the FC, and/or the endpoints on the TX. 

If you have a flight controller without a USD interface, its a bit more guess work whats happening with the controller. Observe the LEDs, and you can usually see if its properly powered up, if its bound to the transmitter, and sometimes also if its calibrated or armed.

Below is a basic walk-through video on hot to configure, connect and adjust flight controller and radio:

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