I have the same problem.
Certain comments are only shown in list view, or when I hold the cursor over the text where the comment is made. Copying the text to a new document resolved the problem. However, this is not because my document was corrupted.
Here's the reason why, in my case, certain comments were only shown in list view.
Like and earlier post mentioned, the problem occurs when there are comments added to text with other tracked changes. In my case, the adding of a comment by another user unintentionally also caused a formatting change. Hence the text had both a comment and a format change. When copying the text to a new document, the format change tracking was lost (not sure why), and the comment is shown. Similarly, the comment will also show in the original document when turning off "Formating" under the "Show Markup" banner.
This can easily be reproduced by typing some text, make a formatting change to one word, and add a comment to the same word. A soon as you submit the comment it "disappears" - although it can be found in the list view. Note that other changes to the text, like spelling etc, does not cause the problem.
To me this looks like a bug. Surely it should be possible to view both a formatting change and comment linked to the same text.