No I wouldnt say that you are a bad owner!!!!!!!!!!!!! But there is somthing going on at that barn,,,,,,, and if the barn owner etc has them on good food, lots of hay and water, has had the vet out etc etc. then they should be gaining some weight,
If not I would deffinatly be looking into it alot closer.
If it where my horses there, I would have "alarms" going off in my head, and if things were not getting any better I would seriously think about moving to another barn
How many times a day are they worked? Do they have hay 24/7 right now? Because horses that are on the thiner side as they are, should have 100% acess to nice hay and water, if they don't the weight just will not go on,,, a few flakes at different times a day in a herd of that size just wont do the trick.
Do they feed round bales? Have shelter ? Come in at night>?
Are there more horses on that farm that are in the same condition?
Maybe get your parents to talk to the barn manager ,, even the vet,,,, Iam sure if the vet is a good one, he/she will have a few thoughts on the matter