I can't find what's the problem

Don't worry, just cut the damaged leaves and be careful with the nutrients you are giving and the pH of the water (5.8 to 6.2PH). sometimes less is more.

Is soil drying back between watering and feedings? Looks like a Mg deficiency, could be from nutrient lockout, ph problem, insufficient Mg being supplied, or overwatering. Usually if it is showing an Mg problem its has been a couple weeks of lacking it, and it takes a little bit of time to fix it. Biggest thing is make sure the soil is drying out between waterin/feeding. When it dries out hit with a heavy dose of CaMg for the next watering. Should stop the problem from spreading further hopefully. Good luck.