Two-bed resistivity of 7 mega ohm.cm drops from 17.3 Mega ohm.cm to 9 Mega ohm.cm

Hi, straight foward to you. How come a two- bed resistivity of 7 mega ohm.cm when goes through the mix- bed it drops from 17.3 Mega ohm.cm to 9 Mega ohm.cm( the lowest I've seen)? We have done Bacteria Fouling and Double Regeneration, still same problem occurs. Please advice. Thank you.

Please split your comment into individual factual sections and expand on it a bit more. I know it is obvious to you what you mean, but it is not to most of the rest of us. When did you have 17 Mohm water and when did you have 9 Mohm water.

What I am trying to mean is a first deionized water that have 7 Mohm of a resistivity from Two bed. The design is a convention type that raw water goes through media, carbon, cation, anion, storage tank then, mix bed and column then point of use.
The point is we are facing a drastic drop of resistivity from the mix bed's outlet. It drops from 17 Mohm to 11 Mohm something although the two bed is still 7Mohm.
We have solved bacteria fouling and double regenerating. Still the problem persist.
Please advise,
Thank you.