Difference in render and render :update

I have a situation where i am using extjs for my tab layouts. When i am getting a tab to be loaded i am making a call to a method and from there i do if(some condition)   render :partial=>'x' else   render :partial=>'y' end

When partial 'x' is rendered, upon some successful completion of an ajax call i then render partial 'y' by doing

render :update do |page|   page.replace_html 'div_id', :partial=>'y' end

Now the issue is in partial 'y' i have an observer and it gets fired if am coming to that partial from x and then y. But if i come to the y partial straightaway the observer is not getting fired at all.

Can someone help me with what could be the difference that is not letting the observer to fire.