Presume you are using latest librepilot , (16.09)
Craft must be still whist arming.
Throttle must be zero. (+check tx throttle curve is at zero )
On the ' rc input tab ' , if you have a channel assigned to a function ( i.e. Gimbal or buzzer ) that is not connected then it wont arm ( set to 'none '. And 'disabled ')
If using switch to arm :
Check on ' rc input tab ' that the switch is assigned and works.
Make a note of settings , then try changing the neutral value by a few points.
If using stick to arm , arm position must be held for a few seconds ( and nothing else moved.
Double check that the saved 'green tick ' happens.
Have a look on system health - FC won't arm if there are warnings there.