Boiling water on the powder
Pouring 100 degree Celsius water directly onto matcha extracts catechins aggressively and can scorch the leaf, which produces a flat, bitter cup. Stay in the 70 to 80 degree range for whisked preparations.
Skipping the sieve
Matcha clumps in storage, even when sealed. Whisking unsifted powder leaves visible specks and grit and dulls the foam. Pushing the powder through a small furui takes 10 seconds and visibly changes the bowl.
Wrong ratio or old powder
Under-dosing produces a thin, watery cup and over-dosing turns usucha bitter. Aim for 2 grams to 60 to 70 milliliters as a default. An open tin that has sat for several months will read dull and hay-like no matter how carefully it is whisked, so track open dates.