Hi US
I looked into your membrane model, its made for high pressure systems with a typical input pressure of 15bar = 225psi.
Thats why your membrane isn't working how you would like it, unfortunately unless you can get your money back you are going to have to buy a booster pump to up the pressure to at least 200psi with a flow rate greater than 20ltrs/min.
You'd probably be better off buying a new membrane, the model you want is the GE osmonics desal AK4040TF or you could go for a filmtec xle4040 membrane (desal is better though).
Sorry to be the bearer of bad news.