A softener sized for the house you bought is not automatically sized for the house you live in now. A teenager starts taking longer showers, a second bathroom goes in, a larger washer replaces the old one, or a pet wash station gets plumbed to the softened side. Each change adds water draws, and sometimes adds draws that run at the same time. The unit that matched the old routine can fall short without any single failure you would notice at the start.
Write down the household as it was sized
If you still have the install paperwork, find the number of people, bathrooms, and water-using appliances the sizing assumed. If you do not have it, ask the dealer or installer, or assume the numbers match the home as it was on the day the unit was set. Then write down the household as it is now. Count daily users, not just residents. Note each tub and shower that gets regular use, and list the washer, the dishwasher, the ice maker, and any other fixture that draws from the softened line.
Measure the daily load, not a guess
Your own water meter is the most reliable input. Read it at the same time each morning for seven days in a normal week, and note any day with a guest or a large laundry load. Subtract the first reading from the last and divide by seven to get average daily use. Your hardness figure should come from your water report or a test reading, in grains per gallon. Multiply average daily gallons by that hardness to get the daily grain load, then compare it with the unit's rated capacity and its regeneration plan. If your hardness has not changed, a change in use is the likely reason the numbers moved. If the supply changed too, use the new hardness figure instead of the old one.
Check the peak, not only the daily total
The daily grain load tells you whether the resin keeps up over a full day. Peak flow tells you whether the unit keeps up in the hour when the shower, the washer, and the dishwasher overlap. A second bathroom often leaves the daily total close to the limit while adding a peak the old unit was never meant to carry. Ask for the unit's rated service flow in writing, then compare it with the number of fixtures that can run together in your home. If the dealer only quotes grain capacity, ask for the flow figure too.
Choose between a setting change, a bigger unit, and a second unit
Not every change needs new hardware. If the increase is small and the unit still has spare capacity under its rated number, a change to the regeneration schedule may be enough, and a technician can set it. If the daily load is over the rated capacity, or the peak flow is too low for the new fixtures, the unit needs replacing with one that handles the new load. A second unit is a different question. It only makes sense if one unit at the right size cannot meet the household's needs, and you should get that reason in writing rather than accept a sales explanation. Whichever path you take, put the options on one sheet: capacity, flow rate, the floor and lid space the unit needs, and what happens to the existing valve and tank.
Re-check on a set list of triggers
Re-check the sizing when a change happens rather than waiting for a symptom. Keep a short list of triggers: a new person living there full time, a new bathroom or tub, a new washer or dishwasher, a new water-using fixture on the softened side, or a change in your water report. Each trigger means one fresh meter reading and one fresh look at the peak. Keep the list with the install paperwork so the next owner or the next dealer can see what the household looked like when the size was last checked.
When you are ready to compare units that can handle the new load, the Water Softener Company Ratings for St. Louis page lists the scores, and the Rating Methodology page shows how each criterion is weighted, so you can see what a score rewards before you shortlist.