N deficiency creeping up.. try to increase that portion without impacting other levels. MAybe 10% boost to N.. start small.
If chlorosis creeping up your plant slows or reverses, you know it was the right choice. Yellow can turn green but any necrotic spots will not heal. The plant may shed some of these leaves rather than try to repair them, so focus on whether or not the progress slows, stops or reverses. .. then, after it's healed, watch out for going too far the other way with an overreaction... happy zone is in the middle.