Loose/less stiff Leather Bellows

[Chris L]

So I seem to be having the opposite problem of most bellows questions on here… I’ve been playing a very nice, all leather bellows concertina (unsure of brand) and over time the leather has stretched and gotten very loose/supple to the point where it’s harder to produce sound. 
 

is there a way to treat or condition the leather to get some of it sold firmness back?

[SIMON GABRIELOW]

Could it be that the card interior which keeps bellows together is the problem here? I just wondered. Maybe this could be no longer keeping everything as should be; just a guess.