The last few minutes are crucial when making brittle. (300 degrees F to 310 F.) It's best to stay with the hot liquid watching carefully not to burn the mixture. Sometimes wet weather can play a role in candy making, as well as rapid barometric pressure change. The science of candy making can get real tricky. The best way to get the desired hard crack is to make the recipe several times allowing for more cooking time, and do a cold water test. Most of all…don't get discouraged if your brittle doesn't turn out the first time.