This souds like a true isolation situation.

Here is your Hogans Heros Prisinors Camp resolution. Good Luck!!

First, make sure you did not leave anything inside the horn. An old reed. A rag. Something to block the air flow.

Look around the outside and make sure the pads look to be closing completely.

Next you really need to know if you have a bad leak, that keeps it from playing. If you have some Christmas lights, thyat may work well enough to get some light inside the horn while you close the keys in a darker room and look for light leaking out.

If that is the case, most older horns used a shellac based glue that would loosen up with heat. If you find one spot that has a leak, use a hair dryer, or a heat gun if you have one as direct to that key as possible. Monitor the heat, as it can damage. You can hold closed the key and work out the leak by closing the key. I often use a large needle or Bari needle spring to slightly pry a pad out to close a leak.

That is about it. If you have a bent key or dent that keeps a key from closing, it gets difficult from there.

Good Luck. I myself am sort of enjoying my newly available practice time.