PX4 RC override doesn't work

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Hi guys, i'm using a pixhawk with ardurover inside. I'm trying to use a ROS node to override the RC channels and drive my rover, through the telemetry. Earlier I managed this problem and i was able to drive the motors of the rover from a ROS node, but now it is not working anymore. I see the right values both on the rc_override and rc_in topics, but not on the rc_out topic, so the rover doesn't move. Do I have to check the GCS_SYSTEM_ID or something similar? Actually, it is already set to 255 (1). Is there a specific "flight mode" that allows the override? I don't know anymore what to do.

Thanks a lot for your help, Flavio