Flights Climbed Not Counting On My Apple Watch Series 6
Why is my flights climbed not counting on my Apple Watch when I climbing up the stairs?
Apple Watch Series 6, watchOS 7
Something could be off with your barometer. It is measuring miniscule changes in air pressure, and low air pressure can presumably throw it off in some rare cases.
Is the Compass app reporting altitude changes correctly for you? Calibration can help you too:
Calibrating your Apple Watch for improved Workout and Activity accuracy – Apple Support