/u/Grunchlk is right. The AD8 comes with a 35mm extension tube. It may be in the focuser and you may have to remove it. Or it may be missing and you have to add it. I forget which is needed for which eyepiece it comes with. But if I recall, you cannot reach focus with BOTH eyepieces without using the extension tube for one of them.
If yours is missing, you'll want to replace it. GSO makes several different sizes, but the one that comes with the AD8 is supposed to be 35mm long.