Opticum Unicable LNB with Vu+ FBC tuners

[Arcy]

I have an Opticum Red Robust Unicable LNB 2 SCR2 LNB (a type which seems quite widely available online) and I want it to feed one of the FBC tuners on a Vu+ Solo 4K running OpenVIX 5.3. The LNB is connected via a splitter (Dur-Line DFV SAT/BK Splitter); this also feeds an Inverto Multibox SAT>IP server, which is happily receiving the upper four SCR channels supplied by the LNB (these are the channels it defaults to).

Now I would like to receive the lower four SCR channels on the Vu+ box, but I can't get this to work. I've connected the antenna feed to the LNB2 input, intending to use Tuner B connected through to virtual tuners C, D and E (I've also tried Tuner A connected through to C, D, E). Unfortunately no Opticum LNB is listed in the menu of Unicable/JESS devices, and so I have configured it as a user-defined Unicable LNB operating on the lower four SCR channels. But the Vu+ box finds no signals at all on these channels, even while the Inverto Multibox is receiving normally.

I do have experience of Unicable (I've previously run this receiver for two or three years with a Spaun SUS 5581 Unicable device as part of a larger DiSEqC system). The cable connecting the receiver and the splitter is OK (it delivers LNB power from the receiver). The Vu+ receiver works normally on its other inputs.

What am I doing wrong?