I would say get Chessie tested for not only PDD, but also ABV and PBFD...And if ANY of those come back as positive, then you definitely have to get the rest of your birds tested as well, because then you have an issue for sure...But Laura is right, you know for sure that Cheddar was positive for PDD, I don't know about ABV or not or if they even tested her, but they are all viruses that are contagious and that they shed regularly, so you just don't know...
The thing that frightens me about Chessie is that her symptoms as you described them are only that her ability to fly has been slowly getting worse and worse over the past few weeks, and that's it...Otherwise she's acting normally...And knowing birds, they hide the signs/symptoms that they can hide, like being lethargic, not wanting to eat, being uncomfortable and in pain, etc., but her ability to fly slowly deteriorating is a sign/symptom that she can't hide if it's due to a neurological issue. That's why it's scary. Usually they won't stop eating, become very lethargic, etc. until it's very advanced, but the neurological symptoms like not being able to control her flying or being able to land, and then not being able to fly at all are things she can't hide..
Again, it could very well be a severe inner-ear/sinus infection, so be sure you talk to the vet about that, they may want to do a regular x-ray just to make sure if they can't tell by looking in their ears, usually it's tough to look in a bird's ears. Keep us posted, and keep a close eye on her, make sure she's eating normally, she's not falling-off of perches, etc.
