I can only assume you have a dual inline TDS meter since you don't say and they should be installed so you are reading the RO only water on the IN probe and RO/DI on the out probe.
I have found from years of experience you can fillter between 3000 and 6000 total TDS per full 16 oz of DI resin so at 16 TDS in you can expect between 180 and 360 gallons at 0 TDS per pound of resin under perfect conditions. Perfect conditions means a 98% rejection rate RO membrane, low or no CO2, normal pH, normal temperatures and 50 psi or higher.
You are using domestic well water so you may have a strike or two against you already since well water is traditionally higher in TDS, more often than not has high CO2, may have other than normal pH and temperatures may be outside normal.
Sediment filters and carbon blocks have very little to absolutely nothing to do with TDS, the yare there to protect the RO membrane which removes 90 to 98% of the TDS and which in turn protects the DI resin which polishes off the final 2 to 10% of the TDS. To capture or remove TDS you need an efficient RO membrane and high quality DI resin.
You have a strike against you with the low pressure, it should be at least 50 psi and 60-100 would be much better. What is your tap water TDS, your RO only TDS and your RO/DI TDS? What is your actual measured water pressure? What is your water temperature? What is youir actual measured waste ratio, not what was claimed when you purchased the system?
What DI resin are you using and is it fresh? If you just purchased it, was it damp and vacuum sealed in a plastic or foil bag? If you have had it awhile did you keep it vacuum sealed in a seal a meal type bag in the back of the refrigerator and is it less than 6 months old? Is it a brand name resin purchased from a reliable vendor or did you get it on ebay or a similar site?
There are huge differences in DI resins and resin quality just as there are differences in RO membranes and how well they work.
