Main Output knob not controlling left speaker (FT Duo)

[spicyhamster]

Hey all, new user here. I've had my Fast Track Duo for a couple years now, but have used the headphone jack exclusively for monitoring. I decided this weekend to finally invest in some decent speakers (KRK Rokit 5). Now that I've got them set up, I've discovered a problem: the Main Output knob isn't controlling the left speaker. It stays at a constant volume no matter how high or low I turn the knob. The right speaker changes with the knob's setting as you'd expect. How do I fix this?

I've tried:
Switching the L and R speakers
Different cables
Direct Monitoring ON/OFF

...which tells me the problem is not the speaker. Is there a setting on the Fast Track I've missed? Any help would be greatly appreciated.

Windows 10
Pro Tools Express (10.3.4)
Avid Fast Track Duo
KRK Rokit 5 (x2)