I think you just need to keep stirring and checking it.
Place a plate in the freezer - to test the jam, drop a little bit on the cold plate, put it back in the freezer and let it chill a little bit.. Then, run your finger through it and see if it has set properly.
I usually put a bowl of ice with a spoon in the freezer and pull it out when I'm ready to test it. Sometimes it's just the right consistency and other times I wait just a few minutes too long...and...glue. I really need to invest in a candy thermometer and just boil it to the temperature where it starts to gel. I have seen THAT magic number out there on some recipes. I had a candy thermometer years ago and when the candy got too hot, it shattered. It must not have been very good quality. Today I am going to make pineapple jam. My son brought home about 8 pineapples the other day.