Another poorly hen

Morning

I've not been keeping chickens that long and I've had nothing but bad luck with them.  It's a long story but I lost my first set of hens.  Then a couple of weeks back my warren had cold and I didn't think she would get better because she was that ill.  Now my speckledy has cold.  I give them poultry spice as one of my girls is moulting and they have apple cider vinegar in their water.  What am I doing wrong? Am I missing something massive and that's why they keep getting ill?  Hope someone can advise.

Thanks
Beth