I suggest that you check the pouch on the rewind unit.  Many of those
that were purchased in the last few decades had plastic pouches that
have broken down and need to be replaced, preferably with leather.  The
symptoms that you stated fit the problem.  The original pouch may have
a hole in it such that when air from a wide open tracker bar rushes
down, the pouch, the valve, and the pneumatic work.  However, when the
hole is only partially opened, as by a hole in a roll, not enough air
rushes down the line to make the pouch work.

Replacing the pouch is not difficult and is easy enough to probably do
in the home, if necessary.  I suggest you do one in the shop first, to
develop your technique and knowledge.

Another possibility is that the pneumatic itself is not supplying
enough power to actually shift the transmission.  Think of the pneumatic
as a lever.  The fulcrum is at the hinge, the force is about one half
the length of the pneumatic and the weight is where the arm pushes on
the transmission.  Try to get the weight as close to the fulcrum as
possible.  I often make a frame so that I can mount it upside down and
backwards so that the arm comes off the hinge end, rather than the open
end of the pneumatic.  If I must mount it in a more cumbersome
location, so be it.  I care more that it works now and in the future
than what it looks like.

One of my observations is that many modern QRS rolls have holes that
are almost too small.  Some bleeds or a combination of pouch leaks and
bleeds result in slow or weak operation.  I have also observed that the
bridges that hold long notes on the rolls together make some valves to
flutter.  The player tries to play each little hole, instead of holding
the long note.  The valves "float" and leak, rather than doing their
proper job.

Jeff Davis - in Seattle, home of green tomatoes!