Well that could mean a lot of things. What has the owner told you other than the horse is not doing good in the barn? Did the owner say why the horse is not doing good in the barn?
1-The horse doesnt like his stall or surroundings-Could be hes a nervous horse-could be a weaver-could be sore-the horse could have ulcers or worms. Could be a number of things.
Ask the owner what specifically does he mean why the horse is not doing good in the barn.