Dual extruder single nozzle printer filament from one side going into the other

Hi there,
First time poster and a newbie question.
I am trying to set up an old Athorbot dual extruder single nozzle printer. The single color works well. However, when I try to print in dual color, one of the extruder never output any filament at the nozzle. Instead the filaments of the other extruder just creeping up the tube of the other and the non-printing extruder just doing a gradual retreat up the tube.
Any advice on fixing this problem ?
Thanks in advance.