Thread:Project:Support desk/My tables don't appear sortable

I'm using MediaWiki 1.22.0rc2 and default template Vector. Everything works as it should. Except one thing: When I create a sortable table, it doesn't appear sortable. My code looks like this:

In read mode, there is no arrows so I could sort the table.

I could not find any information about anything additional that needs to be installed or configured. What am I missing?

The wiki is closed/private, so I can't provide a link...