You shouldn't let them sleep in the nests, maybe your coop and roost space is not big enough to accommodate them?Yes some sleep on roosting bar & one or two sleep in nesting box. The poop in the box was always a typical solid poop with white cap. It's only until recently, that this watery stuff appeared.
Well, definitely a digestion issue.If it is a watery poop, does that mean one of them is sick?
What all and how exactly are you feeding?
When I have a bird that is 'off'....
I isolate bird in a wire cage within the coop for a day or two....so I can closely monitor:
-their intake of food and water,
-crop function(checking at night and in morning before providing more feed),
-and their poops.
Feel their abdomen, from below vent to between legs, for squishy or hard swelling. Check for external parasites or any other abnormalities.
Best to put crate right in coop or run so bird is still 'with' the flock.
I like to use a fold-able wire dog crate (24"L x 18"W x 21"H) with smaller mesh(1x2) on bottom of crate under tray.
Then you can put tray underneath crate to better observe droppings without it being stepped in. If smaller mesh is carefully installed, tray can still be used inside crate.