Interesting question.
I don't believe the issue is that the Church of Christ hasn't presented the true Gospel (granted there are those who don't, but overall), but rather that Natives haven't fully embraced it.
On the rez where I work I've met a few Christians but they aren't really fully committed to the idea. I'm not speaking of their salvation but they like to play games and mix their culture, their traditions, and Christianity and quite frankly they often seem to have something that is a mutation of all three.
I see it and I have to ask myself "Are they reading the same Bible I am?" because in my Bible God doesn't like when you try to have your feet on both sides of the fence. Like it says,
gather or scatter.
Here's what I think the problem is. Those who preach the Gospel of Jesus Christ to the Native cultures of the USA tend to mealy-mouth (they don't speak directly and boldly) it in order not to offend. But the Gospel is offensive! If you can preach Jesus Christ crucified and not offend your audience something is wrong! Desperately wrong.
In 1 Corinthians 2:4 Paul says:
My message and my preaching were not with wise and persuasive words, but with a demonstration of the Spirits power
In 1 Corinthians 1:7 he says:
For Christ did not send me to baptize, but to preach the gospelnot with wisdom and eloquence, lest the cross of Christ be emptied of its power.
So rather than people bringing the true, unadulterated, and very offensive Gospel to native Americans they try to be eloquent and wise. The cross, for those who are hearing the message, is emptied of it's power and there is no demonstration of the Spirit's power.
Then these "missionaries" scratch their head, look about all bewildered and wonder why they didn't have more of an impact. That's easy, they had no impact because they compromised the Gospel in order not to offend people.