The reason why you can't see anything with the 5mm is probably that your optics and skies won't allow you to use 400x. It takes exelent optics and great seeing to push an 8" to 400x. You also can't expect to see an Airy disk unless you have very good seeing. Your telescope may also be out of collimation. Your diagonal may be of poor quality. You might not be allowing the mirrors to cool down. There are many things that may be causing your problems. Wait till a very good night to really test your optics, and make sure it is collimated.