My guess is the outputs on the device are labeled "L/Mono" and "R". Is that correct?
If so, then the left jack has a switch in it that allows the unit to combine left and right channels into one when only the "L/Mono" jack is used. That switch will only be engaged when the plug is all the way in, so pulling it part way out opens the switch and separates L and R.
I don't know if the jack is defective, a cable is defective or the wrong type, or if it's just the effect you're using. I offer this only as a clue that might lead to an epiphany.