This issue below exists, however I think the REAL issue might actually be something to do with the fact that we are using a forms based authentication based sharepoint. I've discovered that if I return the next day to a pdf, the comments don't work, and the resources files all contain some html of the login page!!!
The (old) issue:
In some cases, when a user opens a pdf that has comments stored in sharepoint, the comments and reviewers etc are not displayed, even clicking on Check for New Comments makes no difference.
I discovered that if the files stored in the users profile under Adobe/Acrobat/9.0/Synchronizer/resources (files called resource-[NUMBER]) are deleted somehow, then the next time you open the pdf, the comments will not be displayed and neither will the reviewers. Also, after you've deleted the files, and reopened the pdf, the resource files get recreated but are all 0Kb (empty).
The fix for the issue seems to be to remove all files in the resources folder, then remove the pdf from the review using Track Reviews > Right click pdf > Click Remove Review from Tracker. Close the pdf, then reopen. This time, you will be asked to Connect to the review, and when it has done so, the comments etc reappear.
I can't be sure that the way I've reproduced this problem is exactly the same way that is causing the problem on clients machines, but it seems to produce the same problem.
Does anyone know of a fix for this? Adobe support recommended reinstalling Adobe Reader 9 :|, which is no use at all.