It can’t correctly measure if the temperature of the substrate is too high.
Please measure again after lowering the temperature from the surface of the
substrate such as using water.
It can’t be not formed a proper coating membrane if you apply on not perfectly dried paint surface because of the volatiles generated from the coating under. (Eg: It doesn’t become hydrophilic. Surface resistance unchanged) Please be careful drying of the underlying paint varies with seasonal reasons as well.