Between the transmission and air motor on most Welte Licensee actions
there is a brass turret valve mounted horizontally.  Your problem could
be here, either a cracked or loose tube or a misaligned control rod
that is separating the two sections of the valve, or even a broken coil
spring.

There might also be a problem with the tube that goes down to the
cut-out valve, which is under the reproducing apparatus in the bottom
of the piano.  Hope this helps.

Tim Gautreaux