Here's some background information on amino acids https://www.healthline.com/nutrition/essential-amino-acids

They are the essential building blocks of all proteins, and some of them we need to get in our diet, since our bodies can't make them. Amino acids need to be actively transported from the blood into the brain, and transporter molecules recognize several of the amino acids, so boosting their levels can mean that other amino acids are outcompeted, and very little gets into the brain.