You should indeed up your nutrient concentration. 3-400ppm is way to low for flowering (3-400ppm is generally recommended for new clones and seedlings). Since you are so low, I would recommend getting them up to full strength (circa 12-1300ppm) slowly to avoid stressing them. Try doubling your mixture to get up to around 800ppm, then after a week, increase it again to get up to around 1200ppm or so.

You will want to start using CalMag with your RO water like Weedhound said. Tap water is full of minerals including Mg and Ca (especially Ca) that your plants need. Unless your tap water has a very-high starting PPM (higher than 300ppm), I would recommend using it instead of RO water.