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 => ":assets_host/system/:class/:attachment/:id/:style/:filename",      :path => ":rails_root/public/system/:class/:attachment/:id/:style/:filename",      :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 ?