The reverse osmosis unit is highly dependent on pre-treatment. Over time, any reverse osmosis unit will eventually plug up due to calcium/magnesium/iron...etc... The way to maximize the life of the membrane filters is to ensure that 1) the pretreatment salts are kept at high levels, 2) pretreatment filters are continuously cleaned/replaced, 3) ensure that signifacent volumes of 'flush water' is produced to limit calcification/deposition in the membranes. Purchasing high quality membranes also extends the life of your system.
I hope this helps,