Loose leaf tastes thin and bitter at once
Weak flavour and harsh edges in the same cup, usually with a ball infuser in a mug.
Likely cause
The infuser is too small. Leaf expands more than fourfold, so a packed ball becomes a plug water flows around, and the response is usually to steep longer.
Try this now
Brew the same tea loose in the mug and strain it. If that cup is better, the infuser was the problem.
What to change
Fill any infuser no more than a quarter full and buy the largest basket that fits. A spring-loaded ball holds leaf for about 213 ml against a mug's 350.
The reason behind it
Thin and bitter together always means too little leaf steeped too long.