Had similar issues on our c/f Zona tandem utilizing DuraAce STI. Front shifting was problematic (similar to yours), rear shifting was A-OK.
Fought with the front der. shifting for 3,000 miles. Even stoker Kay got fed up!
Her comment: Hey we've been using barcons for almost 30 years on our tandems . . .
Went to Dura Ace 9 speed barcons . . . now have 14,000+ miles on that Zona . . . problem eliminated!
Lo-tech out does hi-tech in some instances!
Pedal on TWOgether!
Rudy and Kay/zonatandem