I think the problem has to do with the composition of the soil you are using. It would be good to know the composition of the substrate to pinpoint the issue.
It turns out that clay soils are made up of laminar crystals of silicon, oxygen, and aluminum that have very negative charges on their surface. These negatively charged crystals will be able to attract positively charged nutrients, preventing the plant from absorbing them. This will cause a drop in pH (acidification).
The solution would be to either change the soil if you are in the vegetative stages of the plant or, if not, increase the number of nutrients so that they reach the plant.
I hope this helps you. 😊