Getting your jam to set is really a hit and miss. I had found when I first started to make jam's that some set better than others. Here are some of the encounters I had
Some set like concrete
Looks like syrup to me
Some use pectin
Some do not use pectin
Monday the same jam set but not Wednesday
The temperature was perfect but the jam did not set
I had to use more pectin
Weather outside/temperature inside
The list goes on and on and I can only tell you you can go to a million websites on jam and even get an expert's opinion but this is something you are going to learn as you go. Any jam that does not set can become a wonderful syrup or condiment for baking or cooking meal.
The reason can go from the size of your pan, amount of sugar, the temperature and timing. Here are some tips on how to get your jam to set.
* The 1st and easiest way to see if your jam has set is the plate test. It cost you nothing! Take two small saucers and spoons. Put them into the freezer prior to starting your jam. Prior to filling your jars, take the saucer out of the freezer and plop a dabble of jam onto the saucer let is sit a minute. If it is gooey and or runs it is not set. If it doe not run down the plate it is ready to jar.
* Make sure your pot you are making you jam in is a non-reactive pot. And make sure it has
high sides and wide you want to keep that rapid boil that you can not stir down and evaporation is key to good jam.
Temperature is key and the one way I always cook my jam. I use a candy thermometer. I do not recommend using a meat thermometer. And no not the one you use for a common cold. Funny Jammer!
Let your jam come to a steady boil with a temperature of 222 to 223 degrees.
This subject is a long one but I am doing my best to keep it simple for you. I have done more research on pectin then anyone should. But I learned a lot. It is a learning curve that can throw you any time. Not all jam's set alike. Go with the flow and do not let anything stop you. Here is a link to my post on Pectin to use or not. Also a post on Making your own Pectin.