They may have caught a cold, with the sudden frosts and now constant rain,
i have two or three showing the same symptoms, i have infact lost one, a pol maran,
It's one of these things it's better to ask no questions, and sort things out as they come, just keep doing as your doing, poultry spice etc they either will or won't pick up, of course it is frustrating, but there is very little that can be done without vets trips costing money, you could try hot mashes and put out lukewarm water, truying to raise body temp
as a precaution don't buy in/sell birds in-case it is catching, it's better to keep it contained
figers crossed for the best