Moisture is the enemy of kebab, and matcha too. Store it airtight, in a cool dark place, avoid heat fluctuations, so a drawer is good, not the fridge, after opening consume it within a month, I usually do not stock up on matcha, not do the teashops, they keep it in tencha - leaf form and grind it gradually as needed, and pack it, so they do not grind it all ahead, generally 3 month max before you bought your matcha it was in leaf form.
And if you make koicha, sift it, static electricity can make it lumpy, that is not good. P.S. Buy directly from Japan and from reputable vendors.