First check that the drain line flow restrictor isn't plugged. It's either in line of the drain line or inside the RO end of the drain line.

Second, check that the drain line is clear and is routed as straight and direct to drain as possible avoiding any up routing.

Third, check the pressure in the tank and make sure you don't get any water out of the valve.

Fourth, empty the tank and set the pressure at 7 psi static.

Many thanks to justalurker to his/her suggestions.

My drain line flow restrictor was not plugged.
My membrane filter was a few years old as well.
I replaced both membrane and added a matching flow restrictor and everything is good now.

Just thinking, do you have hard water?

If yes, do you have a softener?

If yes, is it working correctly?