sounds like one or more of the membranes are not seated properly or they no good to start with.
while you are producing pure water, pull the pipe out of the product end of each membane and test each one individually.
my tds is over 500 and comes out of the ro at about 4/5,
also what is your water pressure as this makes a differance, mine has a booster pump and runs at 90 psi, although saying that it should still come out better than 58