SOLUTIONS THAT WORK:
I have a MBP 13" late 2012 running Sierra 10.12.2, a Blue Yeti Pro and Screenflow simultaneously on two monitors.
First, I CONSTANTLY have to go to System Preferences-Sound and then go to the "Input". Sometimes the Mic is highlighted, sometimes not. If not, I change to it. THEN even if it is selected, I have to slide the input volume slider back and forth before the Yeti works.
Secondarily, sometimes a Yeti will just shut down the USB ports... like kill them and give you the "The device is drawing too much juice from the port, so I killed it," message. Unplug everything and shut Mac down. PLUG NOTHING into the USB and re-start. Change cables on the Yeti (my problem happens when I plug in the Yeti's cable WITHOUT the mic attached, so it's the cable that comes with the mic). After it recognizes the Yeti with the new cable you can switch back (until it happens again).
Hope this helps!