Which one is correct, "the complaint is still not resolved" or "the complaint is still not sorted out"? And why?

 In my opinion, in the first part of the question “the complaint is still not resoved",the speaker wants to convey the meaning to the listener that the Complaint under consideration is still pending. It requires some more time for reaching some conclusion. It may involve some discussion before finding out its solution.

The part of the question “the complaint is still not sorted out” indicates towards the same meaning but with a difference.

It shows that the speaker is visualising and expecting a quick solution to the complaint. He is not imagining that it may involve some discussion in order to reach a reasonable conclusion. Using ‘sort out ‘ makes the situation informal.