Interesting. All I can say is since your father was seemingly only part-Native American (and it sounds like he was likely less than half Native based on what you're telling us), it's quite possible that you simply didn't inherit the DNA genetic markers for Native Americans. DNA inheritance is random and can vary widely even between siblings. Still, to complicate things even more, despite the fact that Native American markers don't seem to show up in your DNA, that doesn't mean that you don't have Native American ancestry in your background.

It sounds somewhat bizarre, but DNA inheritance is random and while you do inherit half of your DNA from each parent, if your parents are ethnically mixed, the DNA you pick up from each parent can differ from say, the DNA that a sibling has inherited. You may have simply inherited all of the European DNA on your father's side to comprise the half of your DNA that comes from your paternal side. However, the science is still very young in this area and will continue to improve over time, so that if you were to take another test a few years from now, you could have genetic markers identifying you as partially Native American.