Oil in Water analyzer calibration

In DM water generation plant we have multi-stream Oil in Water analyzer(Make: OPAL). Whenever process flow changes the value of analyzer also fluctuates. Means for the quality of water analyser gives the different value at different flow. I have done zero calibration many times but we are getting the same problem. What should I do?
I haven’t worked with these analyzers specifically, but most every analyzer I have they are extremely flow sensitive. How much is the flow changing by? Do you have a bypass flow going to the analyzer? It would be beneficial to see a diagram of how you have it piped in. Seems to me that you will have to concentrate on making the flow to the analyzer more stable.