Salt bridge
A hard crust of salt that stops a softener from making brine.
A salt bridge is a hardened arch of salt that forms in a water softener's brine tank, leaving an air gap between the salt above and the water below. The tank looks full, but because the salt isn't touching the water, no brine forms and the softener can't regenerate — so hard water quietly returns. Break a bridge up by pushing a broom handle down through the salt; pouring hot water over a stubborn one helps it dissolve.