Unfortunately, there is not a simple answer to the OP's question. Factors such as cookware, quantity of ingredients, amount of liquid, temperature of stove, altitude, and even weather come into play.
The issue could be the candy thermometer, and it can be calibrated as mentioned in a previous post. Remember though that water boils at lower temperatures at higher elevations, so unless one is at sea level, it will be off by a couple of degrees. As another poster mentioned, candy making is an art, Keep trying. It takes practice to get it right.