First confirm the feed valve is fully open and the prefilters are not overdue. Then separate membrane production from tank problems by checking whether the RO line produces water before it reaches the tank.
Start with feed pressure and clogged prefilters
Reverse osmosis depends on pressure across the membrane. Low household pressure or heavily loaded sediment and carbon prefilters can reduce production enough that the tank appears to stop filling.
If flow declined gradually over months, maintenance is a stronger suspect than a sudden tank failure.
Check whether the membrane is actually producing water
Close the tank valve and briefly observe product-water flow according to the system manufacturer’s service procedure. Very weak production points toward feed pressure, prefilters, membrane fouling or a flow restriction upstream of the tank.
A normal production stream with an empty tank shifts attention to tank pressure, the tank valve, tubing or the check/shutoff assembly.
Tank air precharge can block water storage
A pressurized RO tank uses an air charge to push stored water to the faucet. Too much air pressure can prevent the tank from accepting much water, while a damaged bladder can create other pressure and capacity problems.
Tank pressure should be checked only using the manufacturer’s procedure with the water side emptied, because a reading taken on a full tank is not the same thing as the empty-tank precharge.
When replacement parts or a new system make sense
An old membrane, failing tank or faulty automatic shutoff valve can each justify a targeted part replacement. If several major components are aging at once, compare that repair total with a newer tankless or replacement RO system before spending repeatedly on an old unit.