Strange attachment_fu problem: thumbnails limited to 1

I am encountering a strange issue with attachment_du: whatever classes
of thumbnails I define, only one is recognized and accordingly
processed when creating a new image.

For example: :thumbnails => {:thumb => "65x65", :big => "548x548"}
just produces a "big" thumbnail and the "thumb" thumbnail simply
doesn't exist.
But even more strange, when I change the order of the thumbs
declaration in the thumbnails hash it still produces just the "big"
thumbnail. Changing the thumbnail class names or size declarations
doesn't have any influence.

Anyone has a clue what might be wrong here?

Here is the attachment_fu relevnat model code:
class Pic < ActiveRecord::Base
has_attachment :content_type => :image,
                  :size => 0.kilobytes..50.megabytes,
      :storage => :file_system,
      :path_prefix =>'/public/photos',
      :thumbnails => { :thumb => "65x65", :big => "548x548" }