Email marketing soft

I have a pair of full grain leather shoes, and I have tried just about everything to make them soft. I have another pair of the same shoe but for some reason the other pair is much softer. I have tried soaking them in water, using chelsea leather food (a conditioner), walking around in them for many days in a row, but for some reason they are not as soft as I would like them to be. Any suggestions on making them really soft without damaging the leather? Thanks