I would recomend checking what you are worming with? as the wormer I usualt use only gets round woems, so once a year we now use a broad spectrum wormer. We had that problem one fall, no matter how much they were fed and how much corn was added, the would not put on any winter fat. finly I asked the vet, she recomended the broad spectrum wormer. but I have also heard that in parts of the country that their is a disease they can get that also causes lesions in their intestions etc, so the don\'t properly digest, they also become very skinny and waste away.
So it is always good to check with a local vet who may be more knowlegable as the what your birds face ad problems.