UR22MKII - Cannot get two monitors to work from Line Output together

I have the Steinberg UR22MKII and two LSR305 JBL Studio Monitors connected via the L and R Line Outputs. One monitor plays and its audio can be increased or lowered but the other monitor, whilst having audio output is barely audible and cannot be increased or lowered via the volume control on the monitor or via settings within the computer. I am using a MacBook Air and trying to use Garage Band.

Both monitors work when connected individually as I have tested both separately. The issue is trying to get both playing at the same time. Does anyone know why this may be occurring and what I would need to do to rectify this? I am quite new to working with audio recording software so any advice would be appreciated.