Most softener failures are loud. A leak, an error light, or a tank that will not stop draining gets attention. A quieter failure is loss of capacity: the unit still cycles and still delivers water, but it softens less between regenerations. The change can be slow enough that nobody notices until spotting on the shower glass or a dull lather on the dishes brings it back to mind.
What causes the drop
Several things can reduce how much hardness a unit removes between regenerations. Resin fouled by iron or other materials does not hold as much hardness as clean resin. Chlorine and other oxidants in the supply can degrade resin over years. The programmed capacity can also fall short of real use after people move in or out, or after a change in water habits. A technician can sort out which of these applies, but you can spot the pattern before a technician is called.
Keep a simple regeneration log
Start a log with four columns: the day, the days since the last regeneration as the control shows it, any change in household use, and any spotting or soap problems you notice. Add the salt level each time you check the brine tank. Over several weeks you are looking for one pattern: the interval between regenerations getting shorter for the same household, or the unit regenerating on schedule while the water feels harder. Compare against the settings you saved when the unit was working well. If you have no saved settings, ask the company for them now.
Compare the softened tap with an untreated one
Test the cold water at a tap after the softener and at an untreated tap at the same time, using the same test method both times. A softened tap that reads close to the untreated one points to a unit that has lost most of its capacity. Do not rely on a single reading, and do not bypass the unit to run the comparison unless you know how to return it to service, because leaving the house on untreated water for long periods has its own costs. Note the results in your log.
Signs that call for a technician
Call a technician if the regeneration interval drops sharply for the same household, if the softened tap reads close to the untreated one, or if spotting and scale return in the water heater or on fixtures that were clean for months. Ask the technician to test the resin condition, check for iron in the supply, and confirm that the brine draw and programmed capacity match what the unit should be doing. Ask for a written note of the findings so you can compare it with your log later.
What not to do first
Do not add salt and assume the problem is solved. Salt keeps the brine tank ready, but it does not restore resin that has lost capacity. Also avoid changing settings to make the unit regenerate less often without knowing why it is losing capacity, because that can leave you with harder water and no explanation.