Customisations to the Content Query Web Part do not appear for 'normal' users

I customised the Content Query Web Part to show additional columns from Announcements lists, but these columns could only be viewed in the Web Part by Site Collection Administrators - 'conventional' users (even administrators of the root site) could only see the default title column of each announcement.

To resolve this, the ItemStyle.xsl and ContentQueryMain.xsl files that I customised needed to be checked in as major versions. I had initially tried to do this with SharePoint Designer, which appeared to show them as checked in, but it was only when I looked at the Style Library using a browser that I could see what the problem was - a mandatory column called "Title" needed to be completed before they could be checked in.

I edited the properties of each file, gave them a title, and then checked them in as major versions.