Front derailleur not shifting down

I recently bought this bike, and everything seems to work fine other than the front shifter. It shifts fine up to the largest chainring, but won’t go back down. The shifters are clicking and working, I can see the cable go slack when I shift down. The issue is that the derailleur doesn’t follow, it stays at the largest chainring. It seemed to move a bit more after some wd-40, but is there anything else I could do? Maybe a worm out spring or something? Thanks!