Originally Posted by San Rensho

Grab the fron der cable where it runs along the bottom tube and pull it as you pedal the bike in a stand. If its shifts, its the brifter. Remember, brifters have trim stops, its not just big chainring/small chainring. Push the brifter lever twice to make sure you are not getting stuck on a trim stop.

If it shifts manually, first thing to do is check the cable tension. Cable on the small CR should be slack, but not sagging loose. To adjust, loosen the cable stop bolt, pull the cable through hand tight only and retighten cable stop bolt.

If the cable tension is properly adjusted and you still have the problem, then flush the insides of the brifter with solvent, WD40 works fine. The factory grease on brifters tends to solidify and prevent shifting.