What version number of the blackmagic desktop video software do you have installed?
That version is 12.7.1. But it turned out my issue was with the PTZ Optics cameras, which output either HDMI or SDI but not both, and they default to HDMI. I changed the camera's video encoder settings from HDMI to SDI and then the blackmagic SDI card got the video signal.
However, I'm now struggling to find a way to monitor the OBS audio input with headphones. I have an MSI gaming desktop, with an M-Audio M-Track Solo converting my mixer output from XLR to USB. I've added that as an input audio source for OBS, which works fine. I get another audio channel from one of the Blackmagic SDI card inputs, from a laptop at the other end. But I haven't found a way to monitor the audio inputs with headphones. None of the OBS monitor settings seem to work.
Apologies if this is the wrong thread for this question, suggestions welcome.