What deficiency could it have?

any type of soilless/hydro fertilizer setup would work well.. 2-3 parts gives you enough mix/match to adjust to what the plant shows (as far as def or tox that form over time). your light mix comes with a small charge of nutes and it won't provide much later on. within a few weeks you probably have to provide 100% of the nutrients, so treat it like a soilless medium at that point. get 10% runoff.. allow a good wet-dry cycle and repeat.

Hi man. I would recommend following the Biobizz feeding schedule for light mix as you seem to have some PH lockout or mineral deficiency. https://www.biobizz.com/wp-content/uploads/2024/04/Nutrient-Schedule-EN-2024.pdf I use only Grow, Bloom and Top Max organic fertiliser with a little Calmag every other feed. Or if you prefer to top dress with Dolomite Lime instead of Cal Mag this will lower PH and raise Mag and Calcium in soil although it will take a while to balance out. Biobizz substrate and fertillisers are designed to work hand in hand to provide all your plant needs when it needs it. Perhaps if you let the pots dry out completley then follow Biobizz schedule remembering to adjust water PH AFTER adding nutrients. I useBiobizz PH down to lower or Baking powder to raise PH without any probs. I have noticed that after adding Grow, Bloom and Top Max my PH is around 5.5 and needs adjusting to bring it up to somewhere betwenn 6.3 and 6.8. It takes a little practice but once you dial it in, any fluctuations should be easily noticed and rectified. Hope ya resolve this soon and get back to lush green leaves before flowering strarts. Good luck dude.