Mic SM7B isn't getting picked up through Mackie Big Knob Studio

Hey yall, can I get some help? I'm trying to do some VO work over a track. I'm using the Mackie as both my Interface and Monitoring system. As a monitoring system it work's great, but for whatever reason my mic isn't getting lvls at all. I got a fethead, and tried it out on another system and It worked great. Can I get a hand thanks!