Nutrient lockout can occur when the nutrients plants needs are available in soil or water (in the case of hydroponics) but the plants cannot utilize them. This can be due to a buildup of salts from over fertilization, or issues with pH (too high, too low).
Nutrient lockout can be avoided when using a semi-hydroponic growing medium, such as LECA by checking the pH of nutrient solution before adding, monitoring nutrient levels throughout the course of growing, and by regularly flushing the medium with plain, pH balanced water. After flushing, it may be best to wait about a week before adding more nutrients.
References
https://hygrozyme.com/what-is-nutrient-lockout-and-how-can-i-prevent-it/