I asked about worming because I also live in FL, but far north of you. My biggest and most common issues with sick birds have been worms and cocci, both of which can cause symptoms like you describe.
The sandy soil, and the warm, damp climate seems to be very conducive to the parasites.
I'm not an expert, and there are LOTS of things it could be, but those would be my first guesses based on my own experience with my own birds.
If you haven't had a fecal done recently, I'd start there. At least it would definitively eliminate that possibility and you can look elsewhere.
Both worming and cocci treatments are relatively simple, if that's what it is.
Best of luck.