So for weak chicks, basically any chick that is still protesting loudly when it’s warm AND dark I always take out from the brooder and make sure they warm up really well on a heating mat. I have one of
these and just fold it over them with a tea towel between, the direct heat against their bodies really warms them up. You can also warm them up with your own body heat, but not everyone’s got time for that, haha. Then I make sure they are fed nutrition on top of electrolytes. Electrolyte are great for perking them up, but feeding themselves still takes effort. I found for the particularly weak chicks especially need that extra boost of energy to make recover.
For this I add some egg yolk to electrolyte solution and feed via a small syringe. You only need to put a drop against their beak, and they should drink it. Go slow, make sure they have swallowed each drop properly before feeding the next. Too much and you might accidentally get it down their lungs.
When they’re ready to run around, I put them back with the rest but continue to hand feed until I see them eating ravenously on their own like healthy chicks should.