If the water tds figure after r/o process has not changed, the problem must be with the di resin, or in whatever plumbing you have after the r/o.
If the water tds figure after the r/o process has changed, then the problem is with the r/o, something i am currently experiencing myself at the moment!.
Either way, i wouldn't worry about a figure of 003-004.
The r/o does the job of bringing tds down from 370 to 030. The d/I does bring it down to 000 but not for very long and I am using enough resin. The system came with Resin and the results were the same if I used their resin or mine.
I also have a 80psi pump which is supposed to help. You get water faster but the results are the same whether you use it or not
If your r/o is only achieving a pure tds of 30 from 370 input, then I would say you need to change your membranes.
These r/o's should be running at between 97 to 99% efficiency. Your r/o is only working at an efficiency of 81%. At 98% efficiency your pure tds after r/o should be about 7 to 8 meaning that your tiny resin canister will last longer.
Whilst I would suggest a bigger di vessel, my first priority would be to replace the membranes. I would start by trying to identify if you just have one faulty membrane by taking individual readings from each output.
I would also consider replacing the 100GPD membranes with 150GPD membranes which will give you more water. Whatever you decide, you should also replace the inline waste restrictor (if you have one) with a new one each time you replace membranes.
And don't forget, you need to replace prefilters regularily. The Carbon block filter is the most important, as it removes chlorine. Chlorine that escapes through the carbon filter causes premature membrane failure.
