Are you using those cruddy "mad minutes" or some computer based assessment program in order to assess whether your students have gained automaticity with their addition facts? If you are, then I guarantee that not only are you doing it wrong, but even worse, you're probably getting incorrect information about what your students know or don't know.
The first thing you're doing wrong is that you're testing all the facts the same, for speed and accuracy. That is just plain wrong: not all addition