Hello dear friend
Maybe poor assimilation of nutrients due to a bad ph or too much fertilizer that starts a lockout and potassium deficiency.
PH is a very important factor to consider, as well as fertilizer dosages. The pH should be adapted according to the stage of life of the plants, so that they can correctly absorb the nutrients present in the nutritive solution.
During the growth period a PH of 5.5 will be ideal for small plants or cuttings, then we will gradually increase it to 5.8. When flowering, we can vary the pH to 5.9 during the two weeks of stretching (especially for indica plants) so that plants have better access to nitrogen and some of the PK, and others micro elements. In this way the plants will have permanently all the necessary food for a beginning of abundant flowering and without deficiencies.
In the flowering phase, the rank of PH will have to fluctuate from pH 6.0 to pH 6.2 for better absorption of the nutrients present. At the end of flowering, the pH can be increased to 6.3-6.4.
An imbalance of PH in cannabis and all plants in general will make nutrients less accessible and force plants to tap into their nutrient reserves in larger leaves.