I had the same problem. Tried to downshift and no pull, after a couple of repetitions it worked temporarily.
The solution was to flush the brifter with WD40. I put the bike in the stand, tilted it so the brifters were at the lowest point and used about 1/4 can between the two brifters. Spray, actuate, spray, several time for each.
Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1