Water that's too hot—like boiling—can cause the matcha to 'cook' slightly, making it sticky and clumpy. Ideal temperature is around 175°F (80°C). If you don't have a thermometer, boil water and let it sit for a minute or two.

Fix: Use a thermometer or just let boiling water cool for 2 minutes. Then pour a small amount over the matcha and whisk gently before adding the rest.