"Author" box disabled in managing spam

I used to go through spam and, at the end of each page, check the “author” box, which then checked all spam comments so that I could delete them at once. For some reason, checking that box no longer works: no comments are then checked. Any idea what could have happened?

I’ve fallen back on going through all such pages, then using the empty button, but it’s still a loss of functionality. Thanks.