If the candy is too soft or doesn’t set, it has not been cooked enough. This recipe really, truly should be measured using a candy thermometer, and if you are using one and still having problems with soft candy, test the accuracy of your thermometer, and double-check that you used the correct quantities and temperature the recipe calls for. If the candy is initially fine but quickly becomes sticky, humidity is probably the problem. Hard candy works best in a dry environment–sugar soaks up moisture from the air around it, and hard candy quickly becomes wet and sticky in the humidity. Don’t attempt this candy on a humid or stormy day. It should always be stored in an airtight container at room temperature, to keep it as dry as possible. You can also buy silica gel packets to store with your candy to absorb moisture and keep it from becoming sticky.