Diagnosis
The air pressure in the blue storage tank bladder is too low. The tank has a pre-charge of air (usually around 7-8 PSI) that pushes the water out. Over time, this air can bleed out or get absorbed.
How to Fix It
Shut off the feed water and open the RO faucet to empty the tank completely. You'll know it's empty when no more water comes out. Close the faucet. Check the air pressure at the Schrader valve (looks like a tire valve) on the tank with a pressure gauge. It should be around 7 PSI. If it's low, use a bicycle pump or low-pressure air compressor to add air until it reaches 7 PSI. Then turn the feed water back on and let the tank refill.
If That Didn't Work
The tank bladder might be ruptured. If adding air doesn't hold or you hear a hiss of air when the tank is empty, the tank itself needs to be replaced.
Heads Up
Never pressurize the tank with water in it. You'll get a false reading and can damage the bladder. Always drain it first.