If the mixture is not cooked enough, the jam will not set properly. This is the only tricky part of this recipe. To make sure the jam is cooked enough, after 20-25 minutes of cooking, take 1/2 tsp of cooked jam on a plate and refrigerate for 3-4 minutes. If it sets, you can stop cooking and remove the pan from heat. If it doesn't set, you need to cook it for a few more minutes and then repeat the testing process until the jam sets on the plate.