I think the range of opinions reflect the fact that not every smart kid who is not living up to his/her potential is the same.
Some suffer from anxiety.
Some have time management issues but are otherwise OK organizationally.
Some really could do the work/focus but they’d rather do something interesting than be bored with homework, studying for a quiz where they “get” 99% of the material without having to learn it, or “show your work”.
Some like being challenged but don’t like to be singled out as the smart kid
Some have various LD’s and have learned to compensate, but once the work in HS becomes more demanding, that gets harder to do without actual accomodations which they are reluctant to use since they didn’t need to in the past.
Etc. YMMV. We don’t know your kid- we can only describe what worked and what didn’t in our own families.