Further to Craig Brougher's answer to Mario Solimbergo, may I add a
couple of points.  Many Aeolian instruments in Europe were made in the
UK, and they differ from the American designs described in most books.

The most important point is that London-built Duo-Art expression boxes
were introduced in the early 1920s, and are significantly different in
detail to the American ones, particularly if the instrument is a pedal-
electric version.

Unlike the American version, British expression boxes were assembled
with gaskets, and it is very easy to unscrew them to look inside for
problems.  Upright versions have the screws underneath the upper seal
cloth (which therefore must be removed to open the box), and grands
have all but one of the screws outside the seal cloth so may be opened
with only a corner of the seal cloth peeled back.  (This is certainly
true for the pedal-electric version, and probably for the electric-
only, although I've never worked on one of them.)

Other than that, the operation of British and American boxes is the
same (although the pedal-electric box looks more complicated with its
additional pair of Themodist accent valves).

Also, after the early 1920s, British-built Duo-Arts and Pianolas often
have 80-note stacks, so do not need note cut-out blocks as described by
Craig.  (I just mention this in case anyone looks but can't find them!)

In any case, where the problem is only with a few notes, I would first
look for a local mechanical answer (piano regulation, valves, pouches,
etc.), before considering the expression system itself.  If the bad
notes are in a cluster, it could be something like an action part
rubbing on the player action, or some part mounted wrongly so it
interferes with the notes.

All sorts of odd things can happen.  One instrument I restored had a
group of notes which would not play, and it turned out to be oil
(incorrectly) used on the wind motor which had seeped down and caused
5 or 6 pouches to go brittle -- all the other notes were perfect.

Julian Dyer