When I got my first horse I had a million questions and concerns, the best thing you can do is ask. If theyre not getting hay I'd ask if they are getting squares or what the grain they are getting it. If it is not sufficient I would move my horse, but I do know that there is a possibility that they are being fed enough but are gobbling it up right away. How have the horses been taken care of since you have been at that stable, riding other horses or you own?
Just ask nicely, if the answer isnt what you want, then respectfully pack your horse up and move him. You have every right to do that and you want a place you know the horses are being cared for.