Can't change headphone volume after Jack Assembly change

I just successfully replaced the Headphone Jack Assembly on my 3GS (thanks, iFixit!). For the first time in months I have both stereo channels.

However, a new problem has cropped up: If I change the volume, the on-screen scroll bar shows up and works, but the volume in the headphones does not change (except that it goes silent when I zero out the volume). The headphone volume is always at midrange, which is a bit too loud for me. Interestingly, If I'm using the external speakers, the volume control works fine.

There are a number of surface-mount components on the harness, and I was concerned that perhaps one was shorting to another component. But I've inserted some insulation in strategic places, so far with no help.

Suggestions?