low ppm as compared to grow schedule

hey all...im running a Humboldt Nute Master A & B recirculating schedule in an ebb and flow but i cant seem to get my ppm at the suggested level. If I am running the suggested ratio of nutes to water is this going to be a problem? I am 2 weeks into flowering and the suggested ppm is at about 1000 but i cant get it past 850. Should I add more base nutes (A & B) than suggested? Or Can i add another more dense nutrient like GH Floranova bloom to boost the ppm a little or will that cause problems? Also, I live in an area w/ extremely clean water and so my filtered tap water comes out at near 0ppm.
 
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