Either a pH issue causing a slight lockout or maybe a defiency of some sort. Start by checking the runoff ph. This should give ya a starting point. If it's reading ok add in some calmag with your watering. 🤘😤
When I had this problem it was solved by adding 1/4 Teaspoon of Epson Salt to 1 Litre of water. I didn't need to purchase CalMag at all. So I just started adding Epsom Salt into every few watering to ensure enough mineral content.