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,