Eventually, the water will reach a point where it can't dissolve more salt. Soap has negatively and positively charged ions. Both the negatively and positively charged ions have to go into solution for the soap to dissolve. Because hard water already has positively charged ions, it has less of a tendency to dissolve any more positively charged ions compared to water that is not hard. The positive ions in the soap can't go into solution very easily, so the soap doesn't dissolve as well as it would in normal water.
Water softeners remove positively charged ions from water so it can perform normally.