What helps jam thicken?

If the jam is not setting properly try adding the juice of a lemon and boiling it all up again for five minutes and then re-testing the set. it You can do this by putting a spoonful or so of the jam onto a very cold plate (taken from freezer). put it back into the freezer for 10 mins. Take it out and push the surface of the jam gently with your finger -- if you see the surface wrinkling up, the jam should set and can be put into jars.