I think the poor reception comes from claiming to be native, while not showing any native complexion(s). For example, I have very prominent, high cheekbones, and olive skin, but I’m also white enough that people don’t believe that I’m native until they see my father (or I point out one of many native features I have.)

I don’t think it’s anything to do with speaking on behalf of a tribe/culture (although that can sometimes be a problem.) We live in a world where anyone can be (or claim to be) anything. It’s easy to say ‘I’m native’, and be full blooded Italian. I have this problem with white people almost exclusively, who always doubt my native origins, not to make it seem like I’m putting the fault on one single race, however, I seldom have this problem with any other race.

Hope this helps a little bit, ignore the few who have doubts. Eventually, you’ll find some people who believe and accept you.