Acrobat X: comments won't show in sidebar list, even though they are there

I've just received a book proof PDF with 302 comments, but only 1 (a drawing markup) shows in the sidebar list. The others can be seen by their highlighting in the text, and the comment pops up when hovering, but they won't appear in the list. I tried importing the data file into a blank version of the same document, and the 302 comments suddenly appear there. However, when I save *that* version, close it, and re-open it, the same behaviour occurs. When I scrolled through the document page by page, the comments then began appearing in the sidebar as I scrolled past each page they were on  --  when I had scrolled to the end, all 302 were in the list. But closing and opening the file again, they disappear and this time won't re-populate the sidebar. The original file was very large (145MB), so I saved it as a reduced-size PDF (18MB) in case it was a RAM problem, but the behaviour is unchanged. In all other files with comments they are still appearing normally. Any thoughts?