Even though you might have Native American ancestry, this does not necessarily mean that this ancestry is included in your DNA and might therefore not appear in your Ethnicity Estimate.
Each of us inherits about 50% DNA from our mother and 50% from our father. However, the percentages of DNA received from grandparents and further back are rarely divided evenly. Therefore, you don’t necessarily inherit the DNA of a given ancestor throughout the generations that have passed.
Since your ancestors’ DNA is not passed evenly to their descendants, one grandparent might have passed on 3% of his/her DNA to his great-grandchild, while another great-grandparent may have passed on 11%.
For example, if you have a Native American great-grandmother, you might have inherited such a small amount of her Native American DNA that it cannot be traced in a DNA test.
In addition, though you might have Native American DNA, it might belong to a tribe that is not part of the sampling used to build our Ethnicity Estimate. There are more than 500 recognized tribes which are ethnically, culturally and linguistically diverse. However the majority of these tribes have not contributed their DNA for research purposes which would allow science to establish better ethnicity models. Therefore, the fact that we did not find Native American DNA in your sample does not mean that you are not Native American.
If your Native American ethnicity does not appear in the Ethnicity Estimate, you might also consider testing one of your parents or grandparents as it is more likely that Native American DNA will be traced in the generations closer to the Native American ancestor.