Ok...how's they droppings looking? Have you got anything like poultry drink to add into they water?..if not disolve 1 tsp of glucose powder into their drinking water..is there been any sneezing or anything else unusual going on?
As soon as you see any of them looking sad try to get into them some extra food & water.
I usually make warm wet mash..with added treats to get them interested of food..eg. peas and/or sweetcorn..if you don't have any mash handy just make it with your normal pellets...cup of treats from freezer

..hot water from kettle...some pellets to soak..few drops of cod liver oil or poultry drink or any other vitamin supplement..wait untill not too hot anymore and let them gobble it up...some times if they are feeling under weather they find eating and drinking too much of an efford or others don't let them ....so tempting luxury meal will get even the poorly one eating..if she does perk up after that..keep going on for several days with mash..
Just try to get them to eat boring pellets after that...

Are they up to date with worming?