S3 monitor inputs 1 & 2 pad/48v not working

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Hi

My new S3 is set up as a standalone monitor controller as per the Avid Youtube video by Collyer Spreen. Everything works except for the controls for inputs 1 & 2 - which is the connected to the monitoring source via the XLR F connectors on the S3.

The issue is that when I enter monitor mode on the S3, the Sel and In keys on inputs 1 & 2 do nothing i.e. they do not change the state of the Pad or 48v. The Pad (Sel key - green light) is permanently on and the phantom power is permanently off. I have tried all possible permutations of Shift, ctrl etc, have connected and disconnected audio cables and have rebooted many times - and cannot resolve this.

The pads (Sel keys) on inputs 3 & 4 work fine.

The S3 firmware is 1.2.0.108 with appears to be the latest according to EuControl.

Am I doing something wrong and/or stupid?

Thanks
Richard