observe_field not working on dependeng collection_select


I have 3 drop downs generated by collection_select, where in the next drop down's option values are dependent on which option value was selected in the previous drop down. This is done using observe_field.

e.g. country => state => city

Here's the code: %div   .label     Grading Scheme Select - Tier 1   = collection_select (:grading_scheme, :id, GradingScheme.all, :id, :name)   = observe_field('grading_scheme_id', :update => 'parent_div', :frequency => 0.5, :url => {:controller => 'grading_schemes', :action => 'filter_parents'}, :with => 'grading_scheme_id') %div   .label     Tier 2   #parent_div     = collection_select (:grade_layer, :id, @parents, :id, :name)     = observe_field('grade_layer_id', :update => 'child_div', :frequency => 0.5, :url => {:controller => 'grading_schemes', :action => 'filter_children'}, :with => 'id') %div   .label     Tier 3   #child_div     = collection_select (:grade_layer, :id, @children, :id, :name)

Initially the observe_field would work on updating child_div (Tier 3)whenever I select a different item in Tier 2. If I select a grading scheme in Tier 1, Tier 2 will update its list. However if i select a new item in that updated list in Tier 2, Tier 3 will not update.

I've checked my local server, and during those occassions that Tier 3 doesn't update i find that ajax isn't producing any calls. I hope someone can help me out on this.