First you should know your PH, maybe your plants are leaking of specific minerals because the ph is to high or to low, but...
I would suggest to get some CAL&MAG (some of them already have som N, enough for flowering). I would start by flushing the pots, make sure that there is no salts buildup, let it dry, then give some cal&mag, and the regular P K solution you may use.
I recommend you to get a good EC/PPM meter, or search if your local water is hard or soft, because it makes a lot difference, for years i had similar issues to yours on middle/end of flowering, when i used soil back on the day, because my city water is 80-150ppm, so there is basically no CAL&MAG.
Even today i give CAL&MAG during flowering (first 5-6 weeks).