This is bound to be something simple, but to get the obvious out of the way...
Yes, I have read the docs.
Yes, the files are uploaded to the correct location.
Yes, style.cfg can be found on the correct url
Yes, permissions on files and folders are correct (files are 644, folders are 755).
Yes, owner/group is also correct.
Yes, the board in question is running 3.3.1.
In fact the new style is a duplicate of one that is already running there, with the exception of a changed style name in style.cfg*. All bits and pieces appear to be present and accounted for. Directory structure has been checked, as have files and byte counts for them. Cache has been purged and pages reloaded.
Despite all this grooviness, the ACP insists that "No uninstalled styles detected". Anyone got some ideas?
*If you're wondering why I uploaded a duplicate: it is about to be heavily edited.