This strawberry jam recipe needs to be microwaved on high power for at least 15 minutes after adding the sugar to ensure a good set. Be sure to stir at 5 minute intervals. After 15 minutes you can begin testing by dropping about 1/2 teaspoonful on a cold saucer. Place the saucer back in the fridge and after a few minutes check if the jam has jelled. I usually find I need 3 - 5 minutes more. Also the sugar and lemon juice work with the pectin in the strawberries to ensure a good set.