I agree with everyone else. We had two older gelding at our barn, who were ribby, but had big hay bellies, and the vet told the owner to do the 5 day powerpac wormer, and lo and behold, within a few weeks, the bellies were going down, and the ribs were less prominant. Fortunately these guys were in a pasture all to themselves, so no other horses got infected, but when I see a horse that's ribby, but has a belly, either she's pregnant, or has worms. And it is true that different wormers are needed at different times of the year based on what types of worms are growing and reproducing at that particular time.
All the Thoroughbreds I've known and worked with have all been a little bit ribby, just cause that's what they are, but they don't have big bellies, and have good muscling. I would talk with a vet, and see what type of wormer you should be using right now, and ask the barn manager what she's been worming them with, and go from there. Keep in mind though, that if you need to change the wormer, because they've been getting the wrong stuff, ALL the horses need to be wormed, otherwise the infestation will just keep cycling, and the horses you wormed will end up getting worms again from those horses who weren't dewormed.