Garage band wont pick up sound form audio interface

So I have an analog synth hooked up through the "instrument" port on my audio interface, which is plugged into the USB port on my computer. I've used this successfully through garage band maybe 5 or 6 times and all of the sudden, garage band wont register any sound coming from the interface. I have the input device set as my interface and the output device set as 'built in output'. The volume on my computer, synth, and interface are all up and the volume dial on my interface lights up when I play my synth. But the volume dial on garage band doesn't light up at all. I've tried using all different cords and I'm almost positive it's not a problem with the interface as it is registering my synth by showing the volume levels. The volume levels in garage band just stay grey and don't register any sound at all. I recorded my voice using the computer speakers, and garage band works fine then but nothing from my audio interface. I updated my computer recently and I thought it could be that, so I updated to the new version of garage band too but still nothing.

Does anyone know whats happening??

Thanks!!