When adding more than one expression order matters! Order them from highest to lowest priority (you can change order by editing each expression).
After adding more than one expression you should test all expressions at once, to check the ordering is correct and no categories are overridden by the wrong expression (test by tapping the 'flask/test tube' icon).
Similar to testing one expression, you need to enter a title of something you would like to be categorized. Then pressing the 'Test' button should return a found category, if testing fails try adding or editing your expressions.