Do you have any idea what it could be?

it's P,K def or lock if ur water more than 140 ppm with CalMagnope 👇

Some of thoughts spots are reminiscent of spider mites. I would be giving that plan a through exam on the under side of the leaves and use a magnifying glass. Part of the plant are showing extra lush leaves indicating over fed with N and some burnt tips. I think you need to get a bit more runoff with your feeds. aim for 20% the pots volume but water in slowly and if needed use a agent to cut the waters surface tension like Yucca. Monitor your PPM/EC as well and start a diary and log your progress. Looks like its been going on for a while.