1) Have you a maker's name and model? Does it use piano type keys or buttons?
2) I had a melodeon with very stiff bellows, and expert advice was to open it up as far as it would go and hang it up open, so that the heavy end is at the bottom. Leave it overnight and see if there is any improvement. If there is winone, try leaving it for a longer time.
3) Playing it regularly and often, will almost certainly cure your problem.
4) Are you certain the bellows are leather? Conventionally, most are made from cardboard.

Good Luck,

Chris B.