They beat up to a greater volume than cold egg whites. Use a heavy-duty, freestanding electric mixer. Beating divinity puts a strain on a mixer's motor. Portable and lightweight mixers may not have powerful enough motors. Follow the recipe directions carefully.
Timing is important.
Adding the hot syrup too quickly or not beating the candy mixture long enough can cause divinity to fail. Overbeating the divinity will cause it to set before you can drop it into individual pieces.