Have their saddles been properly fitted by a professional or at least a very experienced horseperson? If the saddles are too narrow, no padding will make them fit, and any horse has to have their saddles fitted individually, like we get individually fitting shoes. There are too many variables to guess the reason, why exactly your horses are getting sores, so for now I'd suggest you stop riding them and treat their backs, until they are healed (might need a chiro coming out to check their backs, too!), and then get a fitter and be ready to look for new saddles.
I have come a long way, to surrender my shadow to the shadow of my horse.