No audio going into GarageBand
Some solutions found here:

Have you set the mic as the input in GarageBand? (make sure the mic is
plugged in, open GarageBand, click on the GarageBand menu, Chose
Preferences, Click on the Audio/MIDI from the preferences window tool
bar. There will be a pulldown menu called &quot;Audio Input,&quot; click on that
menu, and select the Blue Yeti. )
If your still not getting any audio in from the Yeti, have you tried
setting the Yeti as the audio in in system preferences? if so, do you
get any activity Level Input bar while it's selected? if not, is input
volume set to 100%? (far right end of slide)
Have you tried resetting the cable on the mic side?

If none of this works you might have to send it back as deffective.