Not enough leaf is the usual answer
Leaf quantity sets the ceiling on how strong a cup can be. No amount of extra steeping extracts more than is present, so an under-leafed cup steeped longer becomes astringent without becoming fuller — the harsh fraction keeps coming out after the pleasant fraction has finished. Weigh your leaf once against a scale to calibrate your eye. Most people moving from bags to loose leaf under-measure substantially, because whole leaf looks like much more than it weighs.
The basket is probably too small
This is the most under-diagnosed cause of weak loose-leaf tea. Leaf expands to several times its dry volume, and a rolled oolong far more than that. Confined in a small infuser it cannot circulate and water never reaches most of its surface. Use a basket that leaves the leaf loose, a pot with an integral filter, or no container at all with a strainer on the way out.
Temperature too low for the tea
The advice to avoid boiling water is aimed at green and delicate white tea, and it gets over-applied. Roasted oolong, black tea, dark tea and hojicha all need water at or very near boiling to open at all; at 80 °C they taste hollow and thin. If a robust tea tastes weak, the water was probably not hot enough rather than the leaf insufficient.
Stale leaf, and how to tell
Tea does not spoil, but it fades. Green tea more than a year old, or any tea stored in a clear jar in daylight, will have lost much of its aroma and some of its sweetness while keeping its astringency — which reads as thin and harsh at the same time. Smell the dry leaf: if it smells of almost nothing, no brewing adjustment will recover it.