I second leaving them out in the rain for a while (its the acid, would you believe, in our acid rain, same as in vinegar that helps soften them) then soak in white vinegar before a wash, and also put vinegar in the final rinse. Then fully tumble dry them, bit put a dry towel in to help them dry quicker and to help fluff them up. I know its not very nappy friendly or eco. friendly, but if you tumble them for a few times in a row too they will get softer.
Or rub them together when dry, like youre hand-washing them, although without the water and soap!!
Let me know if that helps and how you manage it in the end.