The first action I ever rebuilt was a 65-note Otto Higel. To be
truthful, I did a partial job -- the valves and pouches were pretty
much left alone. I simply gave the valves the Larry Givens "toothbrush
treatment" and that was all. All the other pneumatics were re-covered
and I used white glue (horrors!). Hey, I was young and I don't think
Art Reblitz had published his book yet.
The stuck note symptom you describe was exhibited by my Higel as well.
It turns out the cause was the beautiful, long nickel-plated tubes that
connect the pouch blocks to the primary valves (I believe it was a dual
valve action). The tubes were rolled and brazed from a sheet of steel
and the seam eventually fails and leaks.
You can either look into replacing them (expensive) or simply slide
a piece of rubber tubing over the entire metal tube. This, of course,
mars the beauty of the action, but it beats the heck out of duct tape
or slathering it with silicone caulk.
I think the Higel action was one of the prettiest ever built. The wood
on the outside was stained and finished rather than lacquered black.
Very solidly built -- a work of art.