MPC Live II - Can't Get Audio Inputs to Work with Line Level

Hi, all-- I'm trying to sample from Ableton live by running the audio outputs from my audio interface (Scarlett 6i6 mk2) into the line inputs of my MPC. I have the 'line/phono' switch on 'line', I have the rec level knob turned all the way up, and the outputs from the audio interface are supposed to be line level. HOWEVER---- I can only get audio to register on the MPC if I come out of the audio interfaces HEADPHONE jacks....so what gives?

Thanks!
Bryan