A water softener trades hardness minerals for sodium, and the salt in the brine tank makes that trade possible. When the salt runs low or forms a crust, the unit can keep cycling on its clock without actually softening the water. A short routine between refills catches that before hard water shows up at the tap.
What to look at
Open the brine tank lid and look at the water level and the salt. There should be water at the bottom of the tank with salt sitting above it. If the water is low or missing, the unit may not be drawing brine properly, which is a reason to call your service provider rather than just adding salt.
Look for a salt bridge, a hard crust that forms over an empty pocket. If the top of the salt looks solid but a broom handle pushed down finds nothing underneath, the salt is bridging. Break the crust gently and note it in your log so you can see whether it returns.
Check that the salt type matches what the manufacturer's manual lists. Do not fill above the fill line in the manual, because too much salt can cause problems of its own.
Set a schedule you can keep
Pick a check interval when you first start the unit, then adjust it after you see how fast the tank empties under your household's water use. Write down each refill and how full the tank was, so the schedule rests on what your house actually does rather than on a guess.
When the check says call someone
Call your service provider if the salt level stays unchanged for a long stretch while you use water, if the water at the tap tests harder than before, if the unit shows an error or stops cycling, or if you find standing water around the tank. A refill will not fix these.
Keep the manual and the provider's phone number next to the unit. Ask the provider at install which parts of this check are yours and which are covered during a service visit.