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?
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 completely impossible that any other animal has gained access to where the food is (any opening 1 inch or larger can be used by many surprising animals) and is helping your birds go through so much feed?
You have another thread that was posted w/in the last 24 hours that stated you are losing birds https://www.backyardchickens.com/t/978697/mystery#post_15235983 - are those in the same flock as the birds you are concerned about in this thread? Looking back through your other posts it seems you have been dealing with some illness and "off" birds for some time now....which only further adds to my suspicion that this is not simply a matter of needing to put weight on the birds.