I think it may have been an issue with the twist in head itself and not the di cartridges. I didn't change the filter again, so it's the same filters I've been using for the last 2-3 weeks with no success but now for whatever reason it is producing 0tds currently. I'm at a loss but I think you're right in assuming it wasn't flowing over the di properly....somehow. fingers crossed it just keeps working and the last few weeks were a fluke.