some factors you might want to consider
1) unless you intend to dilute the solution, accuracy is extremely impt, 5ml vs 7ml per dose makes hell lot of difference
2) alcohol generally evaporate faster
cheers
Might want to check your phosphates as well. Fuel recently shared that for nitrates to go down, phosphates need to be present. Correct me if I'm wrong anyone. I personally have never experienced such a phenomena, however it could be a cause.