He is a young kid who's always been taller, stronger, longer, and more skilled than his competition. He's never been known for his defense and his drive/ motor. It's gonna take him a while to figure it out. That's not Igor's fault, nor is it even Ayton's fault. Big men are notoriously slow to develop and figure things out. That's just how it is with a kid like Ayton.
Think about Doncic for a second- he's used to being the youngest, the slowest, the (physically) weakest player because that's what he's been every time his career took the next step. He's figured out how to be successful years ago because he had ten years to do so. Ayton is the complete opposite. He's always dominated without having to try or work hard. He doesn't know how to do his thing when everyone he plays against is good. In the NBA, everybody is good. Even scrubs are better than what he faced in High School or College.
Ayton will figure it out just like Luka figured it out. Is he ever going to be a defensive stopper? Probably not, but I guarantee you he will be a hell of a lot better than he is now. I was objective before the draft, and Ayton is exceeding my expectations. He is playing better than any reasonable person expected him to play.
This also goes beyond Ayton- it's THE main problem for the whole team. None of them- Ayton, JJ, Oubre, Bridges, Melton, Okobo, and even Booker are fully developed players. There is a reason Suns try Ariza, Anderson, and Canaan for so long at the start of the year. Developed NBA players are better and smarter than rookies, sophomores, or third or fourth-year players. JJ has different strengths and weaknesses than Ayton, but the principle is the same. He is good at A, B, and C, but sucks at D, E, and F. Each of them has holes in their games. That's why the team sucks.
Ayton is not a particularly flawed player- he is just in the early stages of his development.