Salt pellets can cake up and form a salt bridge inside the water softener's salt tank, preventing the brine water from reaching and dissolving salt pellets once salt under the bridge is used up. Problems with the water softener timer, valve motor, rotor valve or venture can also keep the water softener from using salt.
Check for a salt bridge in the salt tank by carefully poking a broom handle down into the salt pellets in the tank. If the broom handle hits a hard crust of caked up pellets before the end of the handle reaches the bottom of the salt tank, then a salt bridge has formed in the salt tank. Break up the salt bridge by carefully tamping down the salt pellets until the bridge cracks and falls to the bottom of the tank. Breaking up a salt bridge will allow brine water to dissolve the salt and the water softener will begin to use salt again.
If you find no salt bridge in the salt tank, then a component failure in the regeneration system is likely preventing the water softener from using salt. Use the diagnostic test system described in your owner's manual to check the regeneration system's components.
If the timer won't respond when you press the control keys to start the diagnostic test mode, then you'll likely need to replace the timer. A broken timer won't run the regeneration cycle that uses salt, so the salt level won't go down.
If the timer goes into the diagnostic mode but the valve motor doesn't run when activated by the diagnostic test, replace the valve motor. The valve motor positions the rotor valve that controls water flow to the salt tank for regeneration. The water softener won't use salt if the valve motor won't move to supply water to the salt tank to make brine.
If the valve motor runs but the rotor valve doesn't move, then a damaged rotor valve can prevent water from flowing into the salt tank to make brine and use salt. Replace the rotor valve if it's damaged.
Finally, a clogged venturi won't allow water to flow to the brine tank. Check the venture and clean it if it's clogged. Replace the venturi if you find damage while cleaning it.