Waterfed cart question.

Hello,

I have a IPC Eagle waterfed cart that is having some issues. I replaced the carbon, sediment, and di resin and the tds is reading 11 - 15. I tested the water coming out of the ro membrane and it is reading 17. It is only taking out roughly 80% of the tds from the source water. I understand that its time to replace however my question is, shouldn't the brand new di resin still be able to lower the tds from a 17?

From my understanding the ro membrane ideally takes out 95% of the the tds from the source water but that the di resin should be able to handle the rest and that the higher the tds going into the di resin shortens the lifespan but doesn't mean it cant purify the water.

Am I wrong to assume the di resin should be lowering the tds of my water even though my ro membrane is only operating at 80% efficiency? I have tried new sponges in the canister, packing it down tighter, and different filters from other systems that are working correctly.

Any information or feedback is greatly appreciated.

Thank you!