Starving for potassium and phosphorous..........and nitrogen to a certain extent, probably due to switching to bloom nutrients too early.
Cannabis still needs adequate nitrogen in early flower and bloom nutrients shouldn't be started until there are small "budlets" developing first and not as early as when the first pre flowers appear or as soon as you switch to 12/12.
Next grow, add a P/K booster to your nutrient line up for the flowering phase, unfortunately it is now too late to "rescue" these plants.