How to recover jam that has set too hard?
The only way I know is by adding some more liquid and warming the mixture, It may or may not improve things...

For what its worth, the consistency of the jam after cooking, tends to be related to the amount of sugar added for a given fruit, with hard skinned fruit typically requiring a slightly different process to soft skinned fruit. It's also controlled by the amount of pectin added to the brew as well as the overall cooking time and temperature.