No Input Signal

I recently got a new audio interface (MOTU Ultralite mk3) and have been setting it up in Reaper to record. I'm plugged into instrument input 1 with guitar and have no input levels coming through Reaper when using the mk3 as my audio interface. The mk3 is selected as my primary audio device in the preferences menu, all input volumes are sufficiently turned up, as well as guitar volume. When using the mk3 on it's own (internal clock source), I'm getting plenty of guitar signal through my inputs and can hear it through my headphones. I can't hear anything when using Reaper. Is there anyone who could offer suggestions to help resolve this issue? Any help is greatly appreciated.

Have you enabled input monitoring?

Yes, input monitoring is enabled. Any other suggestions?