Check the water first
Boiling water is the most common cause of a bitter cup. Above roughly 80 degrees Celsius, catechin extraction climbs sharply and the powder reads astringent and flat. Drop the temperature into the 70 to 75 degree range for usucha and around 70 degrees for koicha, then re-taste before changing anything else.
Then check the ratio
Too little water for the powder concentrates everything, including bitterness. A standard usucha is 2 grams to 60 to 70 milliliters of water. If your ratio is closer to 2 grams to 30 milliliters and you are not drinking koicha on purpose, the cup will read sharp regardless of grade.
Then check the leaf
Open tins lose freshness within 60 to 90 days, and a tin that has sat for several months past opening will read dull, hay-like, and increasingly bitter no matter how careful the technique. Note open dates on the lid and rotate stock so the oldest tin is always your first reach.
Last, the matcha itself
Some matchas are simply built for milk drinks rather than plain whisking, and pushing them into a 2 gram usucha will expose harshness that latte mixing would have hidden. If water, ratio, and freshness all check out, try the same powder in a latte. If it still reads off, the tin itself is the bottleneck.