Hard water does not form lather because Calcium and Magnesium ions present in hard water react with soap to form Calcium Stearate or Magnesium Stearate.

Soap contains sodium stearate. The Calcium or Magnesium Stearate so formed is white precipitate and thus forms scull.

Soaps, when used with hard water, does not lather or produce foam easily because of the formation of a curdy precipitate or scum. This scum is created when soaps react with the calcium and magnesium ions present in hard water.