This is a question we get several times a week. Many people like to make toffee or fudge at home and can't get it to set properly. 90% of the time it's because their thermometer is not properly functioning. To test your thermometer, place about 2 inches of the probe end into a cup of ice water (full cup of ice with water poured over it). Let it sit for about 30 seconds and then read it. It should read 32 degrees F. If it doesn't, it's time for a new thermometer. It's that simple. We've had it happen and I'm sure it will happen again. It's frustrating for sure, but checking your thermometer before you start cooking will save you from candy failure.
Another reason why your candy may not set is the humidity levels in the air. We have a rule, we never cook candy on a rainy or humid day. Even though we keep the shop pretty well climate controlled, we have still had failure on humid days. So, when in doubt, wait until the weather improves or turn the air condition on high for the duration.
One last factor is inaccurate measuring. Cooking candy isn't hard, but it is precise. It's more of a science project than cooking. Measure all your ingredients and cook to the proper temperature. Now, here's the tricky part. When you are cooking candy, the pots will hold some heat causing what's called carry over temperature which may result in burning. We always suggest pulling it off the heat a few degrees under the cook temperature to prevent scorching.
Hope these tips help.