Duo-Art wind motor regulators operate on pressure regulation as well
as volume control. The sprung pneumatic regulator controls the vacuum
level and the tempo slider controls the [air flow] volume.
There is also a reroll bypass in the regulator which allow for
bypassing of the tempo slider control during rewind. If this is
leaking or maladjusted it can have an effect on the tempo regulation
but would probably cause difficulty in setting tempo 0.
The likely causes of lacking sensitivity in the tempo adjustment are
a tempo slider valve that is not seating properly, possibly due to a
kink in the control wire, or the vacuum regulator being too stiff. It
is also possible that the wind-motor or its connections are so leaky
that the system has to be regulated way out of specifications, and this
could eliminate sensitivity too.
If the cloth on the regulator is too stiff from age, or from being too
thick of a cloth, it can make the regulator minimum pressure too high.
The same would be true if anything was preventing the internal knife
valve from closing.
To isolate the problem it would help to determine if the regulator
is actually regulating the vacuum. If it is doing its job then look at
the other possibilities.