An RO tank not filling almost always comes down to one of five things: no feed, choked prefilters, a valve doing the wrong thing, a waterlogged tank bladder, or a spent membrane. Work through them in that order and you will find it without replacing parts speculatively. The symptom to note first is whether you get a trickle at the faucet or nothing at all.
Start with feed and pressure
Check the feed valve at the cold supply is fully open, then check the tank valve on top of the storage tank is open too, because both get knocked shut during work under the sink. Reverse osmosis needs household pressure to push water across the membrane, and a system on a well with a pressure switch set low, or a house at the end of a long run, may simply not have enough to fill the tank at a useful rate.
The prefilters are the usual culprit
Sediment and carbon prefilters load up steadily, and when they do the membrane sees a fraction of the pressure it needs. Filling gets slower over weeks until the tank barely recovers between uses. If the cartridges are past their change date, or the sediment element is visibly loaded, replace the whole prefilter set before diagnosing anything downstream, then see whether the fill rate returns.
Valves, drain flow and the automatic shutoff
The automatic shutoff valve stops production when the tank reaches pressure, and a failed one can stop production far too early or never stop it at all. A stuck check valve on the membrane outlet can let treated water back-siphon to drain. Listen at the drain line: continuous flow that never stops, or no flow at all while the unit is supposed to be producing, both point at this group of parts.
The tank bladder itself
A storage tank holds water against a captive air charge behind a bladder. If the bladder loses its charge, the tank fills with water but delivers nothing at the faucet, and if it is overcharged the tank never accepts water. Disconnect the tank, empty it fully, then check the air charge at the valve with a gauge against the pressure the manufacturer specifies for that tank, and adjust or replace it.
When it is the membrane
Membranes are last on the list because they are the expensive part and rarely the first fault. A membrane that has been fed by exhausted prefilters, chlorinated water it was not rated for, or iron-bearing well water will foul and stop producing at a useful rate. If pressure is good, prefilters are new, the valves behave and the tank holds its charge, replace the membrane and sanitize the system while it is open.
Questions people ask about ro tank not filling
How long should an RO tank take to fill?
Several hours is normal for a standard under-sink unit. If it takes noticeably longer than it used to, treat that as a symptom and check the prefilters first.
Why does the faucet trickle even though the tank feels heavy?
That is the classic waterlogged tank: it holds water but has lost the air charge behind the bladder, so there is no pressure to push it out.
Can low household pressure alone stop the tank filling?
Yes. Reverse osmosis relies on feed pressure, and a low-pressure supply may need a booster pump before the system can fill its tank properly.