sounds like
a) you may need a water softener
b) your membrane is fitted the wrong way around - there is a rubber seal on the inlet side of the membrane, make sure it is on the inlet.
c) faulty or damaged membrane.
Assuming inlet tds of 450ppm, typical rejection should be around 95/98%, therefore outlet tds should be around 22ppm however in practice the higher the inlet tds the higher the rejection %, I would reckon anywhere from 010ppm to 020ppm would be about right.