I'm going to have to make a guess here since you haven't given much detail on what your recipe is calling for, etc.
I assume you are making hard candy by heating a mixture of sugar, corn syrup, and water. I think the trouble may be that you are heating it too quickly.
Try heating the mixture slowly until all the sugar is completely dissolved, stirring until it is. Once the sugar is dissolved, then you can quit stirring and let it come to the boil and rise in temperature until it reaches the desired 300 degrees F.
For anyone else interested in making hard candy, here's a simple recipe to follow:
2 cups sugar
2/3 cup light corn syrup
3/4 cup water
Combine over low/medium heat, stirring until sugar dissolves. Allow to boil without stirring and watch your candy thermometer.
When temperature reaches 260 degrees F., carefully add 1/4 tsp. of desired coloring (more or less if you like). You don't have to stir as it will spread from the boiling action.
Continue boiling until temperature just reaches 300 degrees F. Remove from heat immediately. When boiling stops, carefully add 1 tsp. of desired flavoring and stir through.
Pour mixture into prepared hard candy molds or on to a prepared cookie sheet. By prepared, I mean lightly oiled with vegetable oil.
If you are using the cookie sheet method to just have irregular shapes, oil your hands, too, and take a pair of scissors to the candy as soon as you can bear to touch it. Cut it in pieces while it is still hot and pliable.
I like to sprinkle powdered sugar over the finished candy to keep it from sticking together.
You can store this in an airtight container for a good while. To make lovely gifts you can get yourself some baby food jars and clean them well. Fill them with your homemade candy and place a ribbon around the top or a sticker or label to personalize it.