The pastels you are having trouble with are likely a type that is just hard by design. Many pastels come in varying degrees of hardness. Some are like multi-colored conte crayons, and are supposed to be hard. Some pastels have a hard coating on the outside that needs to be broken in order for the pastel to be used. The best way to test the pastels would be to color on a piece of fine sandpaper and see what happens. If they work, I would suggest using the chalk on a paper with plenty of "tooth" so the paper fibers grabs the pigment. I have never heard of a pastel that gets hard in storage. It is either soft or hard. Most of the harder chalks that I have experience with are smaller than the soft chalk, and seem to have concentrated amounts of pigment in them. They are
generally expensive, so don't throw them out until you have exhausted all possibilities! Hope this helps, Jerry.