If several commits show up and you click on the commit number of one of the earlier ones, rather than the current (top) commit diff tab (because you only want to see the changes in that one commit), you can't make comments. Comments can only be made against the top level diff.

I had made a comment earlier, the author made changes and I wanted
to see just the changes made in response to my comment, not all the changes to that source file.

Of course the workaround is to go to the full current diff and find that spot. Not insurmountable, but it sure would have been nice to be able to make the comments in place.