A softener that worked for years can slide back without any alarm. The unit still runs and the tap still flows, but the house slowly starts behaving like hard water again: a film on the kettle, spots on glasses after the dishwasher, soap that will not lather, towels that feel stiff, and skin that feels tight after a shower. Several of these have other causes, so the first job is to find out whether hardness is actually getting through.
Start with a side-by-side test
You need two samples and a basic hardness test strip or drop kit, which most hardware stores and water treatment shops carry. If you have a cold tap that sits before the softener, such as an outdoor spigot or a utility sink plumbed ahead of the unit, collect a sample there. Then collect a second sample from a cold kitchen tap after the softener. Run each tap for a minute first so you are not testing water that has been sitting in the line. Test both samples the same way and write down both readings along with the time.
If the two readings are close, the softener is not removing hardness, and you have confirmed the problem is real. If the treated reading is clearly lower than the untreated one but still above the target on your control, the unit is working but under-delivering, which points toward its settings, the salt supply, or its regeneration. If you have no pre-softener tap, compare the treated reading with the target listed in the owner's manual or the hardness setting on the control head. A reading that stays well above target through the day is the signal worth acting on.
Rule out the simple causes first
Before you call anyone, check the easy possibilities. Confirm the bypass valve is in the normal service position, because a valve left partly open lets hard water into the house. Look at the control display and compare the hardness number with the figure on your most recent water report or the one your installer recorded. A setting changed by mistake, or a control that reset after a power dip, can quietly undo months of good work. Confirm there is salt in the brine tank.
Then look at the signs that resemble hard water but come from somewhere else. Scale inside a water heater or a showerhead may be old deposits that remain after the softener is fixed, so note whether the buildup is new or old. Spotting on dishes often traces to the dishwasher's own rinse aid setting or a recent detergent change. Rust-colored staining is a different problem from hardness and needs its own diagnosis, so do not read it as softener failure.
Write down what you saw
A short log saves time on the call and keeps a technician from starting at zero. Note when the symptoms began, both test readings, the control setting you saw, whether the bypass was in service position, and any event that came just before the change, such as a power interruption, a salt delivery, a new water heater, or a plumbing repair. Keep the notes on paper near the unit or in your phone.
What to ask when you call
Ask the technician to measure hardness at the treated outlet while you have your readings in hand, rather than simply confirming that the unit is running. Ask what reading your specific unit should produce at its current setting, and how they will check that the resin is exchanging ions. Ask them to put the diagnosis and the measured numbers in writing before any repair or replacement is quoted. If the answer is that the unit needs work, you should be able to say which test showed it.
If you are still choosing who to call, the Best Water Softener Companies in St. Louis ranking can help you build a short list, and the How We Rate Water Softener Companies page explains what those rankings do and do not cover.