I, too, have had a tough go of it with my Polish.
Did you get them shipped? I've heard from a lot of folks that Polish don't do well shipped. Of course, I heard that after I received mine. Sure enough, the Polish was very weak from shipping stress, even though I received them in a little over 24 hours. I had to nurse the Polish pretty much 24/7 for about 2 weeks - syringe feedings and water every 30 minutes, isolated with a heating pad, Nutridrench. I even took her to work with me and kept her on my lap. She also got an eye injury and a beak injury somehow. Not an easy start, to say the least. She's doing very well now even though she's partially blind and will be a special needs chicken for the rest of her little life.
I don't think you are doing anything wrong, but you might try separating the Polish. If you see it start to go downhill, get her warmed up and hydrated. And I swear by Nutridrench. I've seen it perk up babies with 1 drop.