We currently have two video cameras feeding via an HDMI to USB converter into the PC. The OBS software picks these up and uses them without problem but when I try to add them as camera feeds in EW, the two feeds come up as available in the "add feed" option but when selected nothing shows apart from the camera icon. 

Does anyone have any ideas as to what I can try next please. 

Our IT guy has solved it. 

You can't have two different programs using the same camera feed. The first program grabs it and it is not available to the second one.

Good to know.