Possible cause: Bladder pressure in storage tank is too low.
The RO tank has an air bladder in it that helps to push the water out. If the pressure in the air bladder is too low the tank will not push all the water out.
How do I check my RO tank pressure?
Put 7 PSI in the tank and allow it to empty. You may have to refill the air several times until the tank is light weight and has 7-9 PSI in the air bladder.
If the tank loses pressure more than once annually, this indicates a tank failure. Either the bladder within the tank has failed, or the air valve is leaking.
The tank has a 5 year warranty from the date of purchase.