Exposed filter criteria does not show

Hi,

I have a page called 'Library' on my site (Drupal 7 local install). On that page, I have added a views-generated block that lists references to articles and blog posts that I have found on the web on a particular topic.
The content type ('resource') I have specifically created for those references includes fields like author, title, website/publisher, month, day, year, comment ...
The title field uses the link field type and is displayed as a hyperlink to the article itself.
Example: GOLDHABER, Michael H., The Occupy Movement and the Attention Economy, www.goldhaber.org, Jan 9, 2012.

Since the list gets very long, I want the visitor to be able to filter by author.

The block view is therefore set to:
Format: Unformatted list
Show: Fields (inline)
Fields: content: author, content: URL, content: publisher, content: month, content: day, content: year, content: comment.
Filter criteria: Content: Type (= Resource), Content: Author (exposed).
Sort criteria: Content: Year (desc), Content: Month (desc), Content: Day (desc)

But the exposed filter criteria does not show. So, what am I missing here ? What should I check or look for ?

Additional info: I am currently theming the site myself with a Zen's subtheme. But setting the theme back to Garland does not make any difference. The filter does not show in both cases.

I'd be grateful for any hint because I feel completely stuck at the moment.
Thanks in advance,
-- Vincent