I recall servicing a Seeburg L that now and then refused to play.
The problem turned out to be the pouches in the stack.  They shrank
slightly and caused the stack valves not to seat properly.  Once the
valves had seated properly the Seeburg played nicely for a year or so,
and then required another "treatment".

Treatment: Disconnect the rotary pump, and attach a vacuum cleaner
with 60 inches of vacuum, and run your finger back and forth on the
tracker bar for about two minutes.  The extra vacuum is enough to pull
the pouches in enough to allow the valves to seat themselves.

It is worth a try.   Let us know if it works.

Bruce Clark