acts_as_attachment - no thumbnails?

the full-size pics upload OK, but no thumbs or resizing. There are no
errors or exceptions that I can see, just not getting any thumbnails.
The DB column 'thumbnail' is always NULL

rmagick-1.15.6 is freshly installed.

I have the :thumbnails stuff setup per
http://weblog.techno-weenie.net/articles/acts_as_attachment/thumbnailing

using a model like this:

  class EventPic < ActiveRecord::Base
    belongs_to :event
    acts_as_attachment :storage => :file_system, :max_size =>
900.kilobytes, :content_type => :image, :thumbnails => { :normal
=> '800>', :thumb => '150x150' }
    validates_as_attachment
  end

and a controller something like:
....
    @event = current_user.events.find(params[:id])
    (1..3).each { |p|
        pic = 'pic'+p.to_s
        @event.pics << EventPic.new(params[pic]) unless params[pic]
[:uploaded_data] == ""
    }
    @event.save!
.......

(users can upload up to 3 pics at a time to attach to an existing event)

anyone have an idea? is there an easy way to test rmagick (I am not
familiar with it)

thanks
Sam