A softener that appears to be working and is not usually has one of a small number of faults, and they are worth checking in order.
1. The bypass valve is open
The single most common cause after any plumbing work.
The bypass sends water around the softener, and it is opened for service and sometimes not closed again. Check it first because it takes five seconds.
2. A salt bridge
A hard crust across the brine tank with the salt above it and empty space below.
The tank looks full and no brine is being made, so every regeneration flushes the resin with plain water.
Push a broom handle down through the salt. Hitting a hard layer with a void beneath it is the diagnosis. Break it up, remove the loose material, and refill to about a third.
3. Salt has actually run out
Obvious, and worth confirming rather than assuming from a glance — the level can be misleading in a tank with sludge at the bottom.
4. The settings are wrong
Hardness setting not matching the tested water, or not including the iron adjustment. A unit set from a guess at installation will under-regenerate.
Regeneration time set during the household's water use, so the house gets hard water during the cycle and somebody notices it every morning.
A power interruption having reset the clock, so regeneration now happens at midday.
Check the settings against a current water test.
5. A blocked or failed brine system
The brine line blocked with salt sludge.
The injector or venturi fouled, which is what draws brine into the resin tank. This is a common failure and it is cleanable.
The safety float stuck.
Symptoms: the tank stays full of water, or salt is not being consumed between regenerations. A softener that is not using salt is not regenerating, and that is the most useful single observation.
6. Exhausted resin
Resin beads degrade over ten to fifteen years, faster on chlorinated municipal water and faster still where iron is present.
Degraded resin channels and loses capacity, and regeneration no longer restores it.
Symptoms: hardness returning despite correct settings, adequate salt, and a working brine system. Sometimes resin fines appear at fixtures.
The remedy is replacing the resin, which is considerably cheaper than replacing the softener and is not always offered.
The check that narrows it fastest
Is the salt level dropping between regenerations?
Yes — brine is being drawn, so the fault is in the resin or the settings.
No — the fault is in the brine system: a bridge, a blocked line, a fouled injector, or a stuck float.
That single question splits the list in half.
Testing the result
A hardness test kit on treated water settles whether the unit is working at all.
Test at a softened tap, and separately at an untreated one, and the difference is what the softener is achieving. That measurement is worth taking annually, because a softener fails quietly and the household adapts to the change without noticing it.
Water running to the drain continuously
A valve stuck in a regeneration position, a fouled piston or seal, or a control fault.
It wastes a large quantity of water and it is easy to miss, since the drain line usually discharges somewhere nobody looks. Any softener should be quiet and dry between regenerations, and a drain line that is always running is a fault rather than a feature.
Low pressure through the unit
A softener adds some pressure drop, and a large drop points at a fouled resin bed or a partly blocked distributor.
The test is simple: put the unit in bypass and see whether the pressure returns. If it does, the problem is in the softener; if it does not, it is elsewhere in the plumbing or at the well.