Is it a Mayfly 1.1 revB board? or a revA? That station seems to not have the usual 4.3v during the day that most v1.1revB boards get, and is more similar to the voltage one sees on a v1.0 or v1.1revA board. What solar panel do you have on that station, and is it in a very shady location, or is the panel pointed north?
All that being said, did you modify the board for 9v output as I described in this post: https://www.envirodiy.org/topic/need-help-with-clarivue10-turbidity-sensors/#post-17638
We found that the ClariVue sensors work fine with 9v power and the Mayfly can produce a higher current at 9v than it does at 12v, so until we have a better workaround for generating high current 12v supply for the sensor, modifying the Mayfly to output 9v should give you consistent results.