how did you solve your problem?
For any automated retrieval of information, radiometric calibration is advisable. You can do it with the sen2cor plugin. It can be downloaded here: Sen2Cor – STEP
The easiest way is launching it from the command line as described here: Sen2Cor-02.05.05-win64 - AttributeError: 'L2A_Tables' Object has no attribute '_L2A_Tile_PVI_File' - #4 by Fabrizio_Ramoino