Paperclip-av-transcoder gem not generating thumbnails from videos

Hi, I am trying to upload videos in my Rails application using the gem
Paperclip-av-transcoder. The videos are being uploaded to my application
successfully but the thumbnails are not being generated.

          Here's my code:-

   has_attached_file :movie,
     :url =>
     :path =>
     :medium => { :geometry => "640x480", :format => 'mov', :streaming
=> true },
     :thumb => { :geometry => "200x200", :format => 'png', :time => 1 },
     :processors => [:transcoder], :swallow_stderr => false

          Can anyone please guide me why is it not generating the
thumbnails ?