A softener can be installed without trouble and still leave you with weaker water at the taps months later. A drop in pressure usually has a plain cause, and most of those causes can be checked before anyone opens the tank. Start by narrowing down where the problem sits, then work through the parts that restrict flow.
Find out where the weak flow shows up
Open a cold tap at a sink on the supply side of the house, then a hot tap, then a tap at the far end of the home. Compare the flow at each one. If only the fixtures downstream of the softener are weak while a tap before the unit runs strong, the restriction sits between the unit and those fixtures. If every tap is weak, including one before the unit, look upstream first, at the main shutoff, the meter, and the supply line coming into the house. Also note whether the drop came on suddenly or built up over weeks. A sudden change points to a valve or a blockage. A slow decline points to a clogging or scale problem that is still growing.
Check the bypass valve and its position
Most softeners have a bypass valve that lets water skip the unit during service. Find the valve and read the label on it so you know which position means normal service, which means bypass, and which means the unit is in a cycle. If the valve is not fully set to service, the flow can be reduced. Do not move the valve yourself if the label is unclear or if the unit is in the middle of a cycle. Ask the installer to show you the correct position and mark it on the valve or the wall so the next person in the house knows it too.
Look at the screens and any filter upstream
Ask the installer where the inlet screen sits on the unit and whether it can be reached without cutting anything. A screen that has collected sediment will reduce flow across the whole house once it clogs. If you have a sediment filter installed ahead of the softener, a loaded cartridge is one of the most common reasons for a slow decline in flow. Check the filter housing for the cartridge age and the pressure gauge if one is fitted. Replacing a cartridge is a separate job from the softener itself, so confirm who is responsible for it before you start.
Check the drain line and the old plumbing
A drain line that has a kink, a long run with a sharp bend, or a restriction at its outlet can slow the unit's cycle and make the whole system feel weaker. Walk the drain path from the unit to its end point and look for anything that narrows or pinches it. Older homes can also carry mineral deposits inside the pipes. Once the water chemistry changes, loose scale can move and collect at a narrow point, such as a fitting or a valve. If the drop began soon after the install and the pipes are old, ask a plumber to look at the pipe downstream of the unit before you assume the softener itself is at fault.
Ask the installer these questions
Before you call for service, have these answers ready. What is the rated flow of the unit, and was it sized for the number of fixtures and the peak use you actually have? Where is the bypass valve, and was it left in service position at the end of the install? Which screens or filters are on the supply side, and who is responsible for changing them? If the installer cannot answer these from the paperwork, the service visit will take longer and cost more. Keep a note of what you found at each tap and when, so the technician has a clear record to work from.