I have had as basic as a loose connection on the breaker screw that made the ARCfault trip.a tight screw made the tripping go away. Also, a shared neutral with another circuit down stream from the panel.
Faulty appliance may also cause the AFCI to trip.
Isolate the circuit, temporary and safely energize only one half and put a load on there and check results. then energize the other half and put a load on it. then work your way down to the troubled spot.
