If your cotton nappies harden over time it's probably due to one of two reasons, both are curable.
You may live in a hard water area and your nappies have a build up of limescale in the fibres making them crunchy. To dissolve the limescale, on the next wash add a teaspoon of white wine vinegar during the rinse cycle (via the fabric conditioner draw)
Alternatively, it might be a build up of detergent. In the washing section it states only to use half the manufacturers recommended amount of detergent, or less. Rewash the nappies without any detergent until there are no bubbles seen.
If neither of the above applies then it is the drying process. When you remove the nappies from the washer give them a quick shake, to separate the fibres. This will make them softer and reduce drying time.