The problem sounds like it is with the type of bedding or grout used to set the stone. As most people know, water is a natural abrasive. If the proper amount and type of grout is used and SEALED( must be done to reduce ware of gout)all that should be necessary is periodic maintenan ce(resealing).If this procedure has been followed then the problem exsists with the material compatability and not due to lack of proper maintenance. With out seeing first hand, this is what my experiance tells me.