Everything tastes the same to me
You can tell tea from coffee, but two green teas, or two oolongs, seem interchangeable.
Likely cause
Almost never a defective palate. Tasting one cup alone asks you for an absolute judgement, which nobody is good at, and hides the differences you would spot instantly in a comparison.
Try this now
Brew the same leaf twice, once at eighty degrees and once at boiling, and taste them side by side. If you can tell those apart, your palate works.
What to change
Taste in pairs. Start with pairs that differ obviously, then narrow the gap. Two or three cups per sitting, because accumulated astringency flattens everything after that.