Without a cable, the gear where the cable head goes into is pulled by weak spring when the ratchet is released (downshifting). Inserting a cable introduces drag, given that a steel cable is not that flexible enough to bend on the small radius of the gear without resisting bending. To overcome this, the FD's stronger return spring is the one that pulls on the cable to return it to the original position.

Insert the cable and pull on the cable when you downshift. Then try installing the shifter and the FD and see how it goes. It's probably just a very weak spring on the shifter.