Mary wrote:
I'm talking about the shocks on the "stair stepper" exercise machine
but they seem to be the same as those on a car. Anyway, what causes
shocks to wear out, to lose their resistance, when there's no evidence
of fluid leakage? Thanks.

Likely the piston is plastic and it deforms or hardens.

I have a solidly built rowing machine that has that problem right now
and I can't find a replacement.