The conductivity of purified water

My company changed RO membrane column type from LC-HR30-4040 to BW30-4040 for Purified Water system. There are two stage RO (Stage 1: 3-2-1 and Stage 2: 3-1; inclunding 2elements in vessel). However, after set up system, and operated for 2 months, the conductivity of stage 2 did not decrease (The value is about 6.37)

So, hopefully, give me a reason why? And give me a solution for this

Check separate the membren outlet water quality and make sure you want to get your answer