I am pretty sure your problem is the same as mine.
https://www.blackmagic-design.com/support/detail/supportnotes/supportnote/?sid=3945&pid=4012&os=mac&isSDK=0&snid=9700
Click above for more details, just a note though i had to try a number of different cables because for some reason they did not work even though they were stereo jacks.
One thing i did notice is that if you didn’t put the cable all the way in it worked, confused yeah i was too.