If they are truly consuming 1/2 pound of feed per day/bird and are still significantly underweight there is something else at play here. What brand of layer feed are you using and what is the protein content of it? Are you certain they are the ones consuming all of that feed - meaning is it...
When you say they aren't currently eating well, how much are they currently taking in? What are you currently feeding them? What is their body condition? Have you examined them or treated them recently for internal or external parasites?