You've got a phosphorus deficiency and most likely a potassium deficiency as well here... When you say you flushed your plants, I don't think you actually did - it sounds like you just gave it a good drink of water so that you had some runoff?? Flushing means to run 3x the pot volume of pH'd water through each pot... since you're growing in 15L pots, that means running 45L through each one... this gets rid of built up nutes and salts that are most likely locking out the phosphorus and potassium. At the end of your flush, give your plants 1/2 strength dose of the nutes you've been giving her... add a tsp of calmag (it's just another liquid nutrient that you should definitely have on hand). Then let her dry out completely before resuming your water/feed/water/feed routine. While you're out getting some calmag, get a PK booster as well... because these ladies desperately need more of both P and K ... a good NPK to look for would be something like 2-45-28...
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Good luck!