Scriptaculous effect not working..

Hey so here is my code:

<% if @error == true %>
  <%= javascript_tag "New Effect.Shake('form_holder'); return false;" %>
<% end %>

I know that the scriptaculous library is being included in the head of
my view, but for some reason the effect never works. Im using the effect
to notify the user that their login was incorrect, hence the @error
variable. That variable is set in my session creating action and upon an
incorrect login the new action is rendered (there is nothing inside this
action). However, when i make an incorrect login and the new action is
rendered, the source code contains the javascript, just nothing happens!

Many thanks,