It appears that the air finds a way to get out of the bellows somewhere where it should not.
If it is an lachenal or another brand of old english concertina's there may be lots of causes, such as
- blocks to hold up the reed plate have loosened.
- a reed or action plate may be warped,
- a crack in the action plate
- shrinkage of the reed plate, letting the air go around it
there may be more, and I suspect that answers may already be available somewhere in this forum...