Can't shift to small chainring While riding yesterday I could shift into the big chainring, but to get back to the small chainring, I had to dismount and press on the chain with my hand (I'm not talented enough nor inclined to learn how to do this with my shoe while riding).  It's an eight year-old triathlon bike and there was plenty of slack in the cable when I tried to shift.

What could be wrong?
The derailleur was sticking.  A bit of lubrication and all was good again.