Diagnosis
That’s almost always the flow restrictor doing its job too well, or more likely, one of the pre-filters (especially the sediment one) is clogged up and choking the system.
How to Fix It
Yeah, this one's common. First, shut off the feed water valve and the tank valve. Pop out the three pre-filter housings (the first three big white canisters). Check the filters. The first-stage sediment filter is the usual culprit. If it’s brown or gunky, replace it. suggests every 6 months, but it totally depends on your water. While you're in there, check the carbon block filters (stages 2 & 3). Reassemble everything, making sure the O-rings are seated right and you hand-tighten plus a quarter-turn. Turn the water back on slowly and check for leaks.
If That Didn't Work
The actual culprit might be the flow restrictor itself in the RO drain line. If it’s faulty or has debris, it can restrict inlet pressure too much. You can try flushing it with water or replacing it (it’s the small blue plastic piece in the drain line tubing).
Heads Up
When you change the pre-filters, you have to run water through the system for a good 10-15 minutes to flush out the carbon fines before you drink any. Otherwise, you’ll get black specks in your water.