OK, you're on the right track by giving attention for the quiet but, not just after a barking session....whenever she is quiet.
Socialization is much more than dog on dog interaction. It's teaching them how to cope with scary sights and sounds without barking, running away in fear...teaching them confidence. I suspect that part of the training was missing.
Most barking is because the dog doesn't know what else to do or how to handle the situation so, they just bark their heads off. Telling them to be quiet isn't real effective as you've discovered...it's not a 'real' solution to the 'problem' in the dogs eyes.
You can teach her to relax by taking over the 'problem'...physically stepping in front of her and the scary thing. That shows her that you're handling it now...she can relax...her job is done. Thank her for the alert and now ask for Quiet. This isn't a quick fix. It takes some time for them to understand you're taking over but, it goes fairly quickly if you're consistent about it.