Building an Ian Canada Transport : Problems

As I have a good amp with a good - but slave - DAC, i wanted to have a go at making a good upgradable transport.

I have a Station PI, it is powered by two LinearPi supplies, one each side. The Pi is a Pi5, the transport a TransportPI (I think there are different ones. I think I have the last iteration).

It runs Volumio, and the I2S is configured as a HiFiBerry Digi+ Pro, as described in the manual.

The Volumio inteface shows the music playing, but I'm getting zilch out of the transport (yes, the led says it's powered).

Where should I start troubleshooting this ? I'm at a loss...

Thanks,

Pete