[Question] USB device exists... but can't seem to get MIDI signal coming in? Have any input? (Pun intended)

Basically, the problem I'm having is I have a completely stable El Capitan build that has been running flawlessly for ages. My soundcard works fine, all my USB peripherals work great... except the ones that deliver MIDI information. I have an M-Audio Axiom Pro 49 that absolutely shows up under USB devices in System Information, but never registers any MIDI data in MIDI Monitor or the like. Tried different cables and different ports, and I know the hardware isn't the problem because I've used the keyboard on my real MBP without any issue. There's also nothing funky under Library/Audio/MIDI Drivers. Does anybody know what might be the root problem here? I saw one other post here, and a couple on Tonymac, but nobody seems to have a solution.