So I have a pair of ST-6600 integrated shifters for a double crankset, and I've encountered a problem I haven't seen before. The left shifter works perfectly when no cable is installed. It feels crisp, and you can see that where the cable head goes (not exactly sure what it's called) moves cleanly to both positions as it should. However, once a cable is installed, I can upshift once and the shifter will not downshift. The cable head is properly seated where it should be. There doesn't appear to be any obstruction in the shifter, and at least visibly everything looks fine. I have given the shifter a good WD-40 flushing and oiled it as best I could. This is with a brand new, Shimano shift cable, and it happens whether the cable is under tension or loose.

I am stumped. I have come across STI's that simply no longer work, but not ones that seem to work fine without a cable. Does anyone have any ideas?