Tea tastes thin and dull, like almost nothing
The cup smells fine but tastes weak and flat, even with more leaf, a longer steep or hotter water.
Likely cause
Very low-mineral water, distilled, reverse osmosis or heavily filtered, is a common cause: stale leaf can flatten a cup too, but flatness across different teas points at the water.
Test it at your sink
Taste the water on its own, plain and unheated. If it tastes of almost nothing, that same blankness is likely carrying through into the cup.
What to change
Brew the same tea with ordinary tap or spring water instead and compare; a livelier cup points straight at the water.