OK, have answered my own question. I had enabled the 'Limit decoding unless required' option for all cameras hoping to reduce the CPU usage. It did drastically reduce CPU usage but it also disabled live view for all except a selected camera. Will have to live with the high CPU usage.

Hoping someone can put me straight on this. Previously I had Blue Iris configured so that all cameras (x8) were showing live all the time. Now, and I don't know why, only the camera that is selected will show live view. Any ideas?