Feel their crops....are they squishy and soft or do they have a hard lump at the bottom? Soft and squishy can be a sign of sour crop, hard ball can mean impacted crop. A sour crop can come from a simple imbalance of the healthy bacteria, something they ate or a secondary issue. Impacted crop is generally from something they ate that is not breaking down.
If you suspect a sour crop, you can put raw unfiltered apple cider vinegar in all the chickens water. 3 tablespoon per one gallon. The unfiltered stuff has health benifits that the filtered stuff does not have. However if you don't have the raw stuff, the grocery store stuff will work til you get the good stuff. The ACV will not allow yeasts and fungus to live in the crop, which is what generally causes a soured crop.
If you see them squiggling their necks around with a soured crop, they are uncomfortable and want the nasty gunk out of their crops. So hold the chicken like a football in one arm, lean them forward, (never backwards) and massage the crop till the gunk comes up and out the beak of the chicken. Do not hold them in this position for more than 10 seconds so they can breathe. You can do it again so see if more will come up. You will never get it all out, but it will help them feel better.
Offer only soft foods for a few weeks, along with the ACV. No seeds or hard foods that require the gizzard. So things like hard boiled eggs, plain yogurt, cereals, anything that disolves in water. Sour crop does not heal really quickly, and can take a few weeks to heal.
If you suspect an impaction, do not offer any food for 24 hours. Squirt about 1 teaspoon of olive oil down the throat with a syringe. Massage the hard lump for about 15 mins. Do this several times aday in attempt to break up what is in there. Some folks have actually operated on an impacted crop, I have not. But some have had good success with it. You can look thru the archives to see how it is done if you can not heal it up yourself the easy way.
If the birds seem cold, which if the probably are since they are not eating much, keep them under heat at night. A cold bird will not move the crop or digest an impacted one either.
Good luck!