Same thing happened to me last night on my second batch of damson jam - first lot was perfect, really thick and tart and set a treat.
This lot is more like damson compote. And it was my own fault - I overfilled the pan and was to scared to give it the berries on the heat, so it didn't get the proper boil.
I'm going to split it into two lots, reboil with a little more sugar and some lemon juice and see what happens. In the worst case scenario, I have 6 jars of fantastic damson compote. In the best, I have 6 jars of tasty damson jam.