When you load a review, Reviewable picks some default diffs that it thinks you might be interested in seeing. Sometimes, this can result in no diffs showing at all, because you're the review's author or because all files have been reviewed (by someone) or there are no unresolved discussions. This can be confusing, especially to new users.
A few small changes should improve the situation:
First, if no files at all are showing, an information bubble in the file area explains why and offers a button that will show a full diff of all files.
Second, whenever you're looking at an empty diff of the latest revision, a small button in the Changes box will offer to show you the full review diff instead.
Finally, for multi-party reviews "Include changes in files previously reviewed only by others" will default to true, unless your personal setting (based implicitly on the last time you clicked the toggle) or the repo's (hidden!) setting says otherwise.