I need some help again

My gouldian juvies have been in and out of illness, constantly. It's quite stressful on them and myself because I'm not sure what in the world is going on.
A few weeks ago, I JUST treated them with Amoxitex and everything cleared up. However, a week post medication, they fell right back into an illness. One of my babies even died!
Here's my original post when they first got sick: http://www.finchforum.com/viewtopic.php?f=4&t=25350
416 is alive and molting. Seems to be unaffected by whatever is causing the illness. 417 is ill. It's not severe but her vent is wet, wings droopy at times and bobs her tail up and down, as if to shake the excess liquid that comes out when she defecates. My female BB is unaffected as well.
I'm redoing the whole QT process. In their drinking water, they have Amoxitex, Ronivet 12% and Vitalize. I've started treatment last week, on Thursday. But I see little to no improvement. Her vent is still pretty wet, as opposed to before, when I did see signs of improvement, her vent cleared up within 2-3 days. I'm getting worried. Not sure what else to treat them with

I was told on a FB group that it might just be stress and I should just give them vitalize and probiotics and let nature take care of it by strengthening the "good" stuff in them. Think that'll work?