This WAS baffling me completely. I'm posting in case anyone has the same problem, because I spent 3 days troubleshooting before I found the simple issue.
I'm using a valve mic (WarmAudio Wa-251) plugged into my BigKnobStudio to record in my DAW (Logic Pro X) on an iMac. This has all worked fine previously.
Now, I hear the signal from the mic via speakers /phones connected to the BigKnobStudio, and its own meters show a healthy input; but the record armed track in Logic shows zero level, zilch.
I switched to the iMac's internal mic in Logic's Audio Preferences, and then I see plenty of level arriving at the armed track. I also tried connecting a simple USB mic-again, signal coming into Logic. So I believe the fault is not with my DAW (also I haven't changed any Logic Audio Settings since last recording, anyway). I also checked using Garage Band and got no signal coming in there either.
At first I thought my BigKnobStudio might be faulty, but I have a second unit in my other studio, and when I connected it the problem is exactly the same.
So: I FINALLY found - it was the USB connection! I had the Interface plugged into a USB Hub. Connected it directly to the iMac USB sockets and presto, life is good and there is signal in Logic. So just a reminder: checking all cables includes not just audio connections, but ALL connections.