Moon 350P input selector skipping digital inputs

Just bought a Simaudio 350P preamp on Audiogon with the digital and phono stage optional boards. When cycling through inputs, until shows cd, a1, a2, ph, b1, b2. But instead of walking through d1, d2, d3, d4, the input selector skips back to cd.

Checked that the digital board is installed and all connections are solid. Digital source connected to unit via USB recognizes it as 350P.

Is there some kind of reset that will make the digital inputs show up?
They may be designed to NOT show up on purpose UNLESS they have an active digital signal present at each one.
This would be the most likely scenario as to why they do not appear as you scroll through them.
You may be correct, Elizabeth. I have tried several different digital sources (usb, coax and Toslink) and I'm still not able to select any of the digital inputs.

The digital sources are feeding signal to my old receiver so I should think there's nothing wrong with the source.