How much pectin is in the fruit that you are using to make jam, jelly or preserves?
Some fruits naturally have more or less pectin than others. For example, if you are making strawberry jam, you will need to use more pectin (of any kind) than if you are making blackberry jam, since blackberries naturally contain more pectin than strawberries.
Generally speaking, the more ripe and wet the fruit is, the lower the pectin levels are.
You can test for pectin levels and add or subtract by testing the jam while you are cooking it.
As you make the first batch, and are ready to fill the jars; remove a spoonful of the jam, and hold an ice cube against the bottom of the spoon to cool the jam. If the spoonful sets to your liking, you can fill the jars.
If the spoonful does not set, add another cup of sugar, 1/4 cup of lemon juice and more of your pectin, bring to a full boil for 1 minute, and test again!
Pectin levels in fruit
Grapes (Eastern Concord)
Plums (not Italian)
Citrus skins (oranges, tangerines, grapefruit, lemons, limes, etc. - the pectin is high in the skin but low in the fruit)
Grape Juice, bottled
Group III (lowest)
Grapes (Western Concord)