I've had a CAM870 with the same issue out of the box, but unlike yours only with the trigger assembly fitted. I'll share my solution in case it brings you any help.
I had to gently file down the angle shown below on the rear of the cradle arm that engages with the trigger assembly. I had to round it very slightly on the outer edge to get it to release smoothly when each shell was loaded and the arm was in its furthermost front position as it was just catching and preventing it from resetting properly.
For completeness here is the opposite mating part linked to the action bar lock
