Hard landing, IIRC, is clinically defined by a 2g+ landing force. It's unlikely that an airframe that had a hard landing that could cause physical harm would take off next flight without an inspection. It's possible that a FA could have struck their head or body against something during a firm landing. But if all the FA were saying this....maybe it was. Maybe a pilot on the forum can tell us if the A320 series or the 737 series is equipped with a sensor to detect a hard landing?
What was the weather situation? If there was a crosswind or runway condition, pilots are trained to use a firmer landing to avoid floating.