Oriol

That works for clay that is only slightly dry and it is what makes properly stiff clay sticky on the outside and crumbly in the middle, which sounds like what you have been fighting. Wet hands for mild, holes and overnight for stiff.

You have. Open a month, folded rather than sealed, next to a radiator: it has lost moisture before you started, so it cracks the moment you ask it to bend. The tell is that it splits at the beginning rather than after a few minutes of handling.

To bring it back: poke several holes into the lump with a pencil, drip a few drops of water into each, wrap it tightly in plastic and seal that in a bag. Leave it overnight so the water spreads evenly, then knead it properly before use.