I've heard from a few readers who have had a difficult time getting their jam to 220 degrees F (104 degrees C). If you're jam isn't reaching that temperature, don't be afraid to turn the heat up a bit. If you do turn up the heat, be sure to stir it more often so the jam on the bottom of the pan doesn't burn. It's also important to make sure you're using a thermometer with the correct temperature calibration. If the temperature reading on your thermometer is off, it can be difficult to know if your jam is getting to the right temperature.