Channel mixer and multitrack

[pandalikesheadphones]

Hi! I'm having some issues with Channel mixer and multitrack. 

I've got a mono file that I've converted to stereo (Edit > Convert Sample Type). I then amended Right/Left balance on some sections of the track with the Channel Mixer effect. It plays just fine as a separate track, I can hear the shifted R/L balance, but when added to the multitrack, the stereo file sounds like mono: the parts I've amended have an equal sound in both channels (though quieter). What am I doing wrong?

Thanks