Well you never said if its 4040 (40 inch membrane) if it is you need to set it up with your tap pressure for best performance.
Close the tap a little bit so some pure water is coming through the pure pipe and then measure your Tds .. then close it a little more and measure your Tds level and repeat this untill you get your lowest TDS and that is your optimum setting. If you close it too much it will start to rise again.