It sounds like you're doing all the right stuff. If they're eating, drinking, and pooping, then those are really good signs. Do they act like they don't feel well? Are you getting any eggs? (Sorry, I can't remember whose chickens have all started laying and who's haven't) Since they free range, is it possible they could have eaten anything else bad, like maybe rotten food in the compost? Do they smell bad? A lot of chicken illnesses have a particular smell.
It is possible to have more than one egg bound chicken, particularly if the problem is dietary (not enough extra calcium, or not enough water), but that doesn't sound likely. Plus, you'd probably notice that they were trying to lay an egg, and getting distressed because they can't.
But really, as long as they're eating, drinking, and don't seem uncomfortable, I'd just keep an eye on it, and try not to worry too much. You probably also want to be careful with the antibiotics. That can cause digestive distress. I had to treat a couple of hens with respiratory illness, and the antibiotics actually caused them to stop eating. It can give them a stomach ache, plus it leaves a bad taste in their mouths, which makes them reluctant to eat. (At least that's what the vet told me) I had to force feed them until they were well.
