Rails 4 - ActiveAdmin layout is blank but application layout works for custom controller action

I have a Rails4 app with active_admin for admins only.i have been using it for quite a long time and has customised it to the way i want and it has always worked but now , it is this little weird issue which i am unable to resolve.

i have a custom action in my controller block inside my admin/hall.rb model, which is also getting executed as per the server logs which also includes the active_admin layout too BUT on the screen , i cannot see any layout and its just blank but when i use application layout explicitly in the controller block, it works.

ActiveAdmin.register Hall do

controller do


  ##explicitly add helper to be used by existing partials 
  helper ApplicationHelper 
  ###DO NOT WORK
  #layout 'active_admin' # <-- here
  ##THE WEIRD PART################
  layout 'application' , :except => [:activities] ===> this render the application layout perfectly which i dont want , as i need the active_admin layout
  layout 'active_admin' , :except => [:activities]  ==> also render the application layout...STRANGE
  layout 'active_admin' , :only => [:activities]  ==> render the application layout but with only icon and entire page is blank


  def activities
    @hall = MODEL.find params[:id]
    @activities = @hall.get_public_activity
    ##EVERYTHING BELOW THIS DO NOT WORK, I HAVE TRIED IT
    #render :activities, :layout => false
    #render :partial=>'admin/halls/activities', :layout => "active_admin"
    ##render "activities", layout: "active_admin"

  end 




       #######server logs
    Processing by Admin::HallsController#activities as HTML
    ...
    ...
    ....

       (1.7ms)  SELECT COUNT(*) FROM "activities" WHERE "activities"."trackable_id" = $1 AND ("activities"."owner_id" IS NOT NULL)  [["trackable_id", 1021]]
      PublicActivity::Activity Load (1.5ms)  SELECT  "activities".* FROM "activities" WHERE "activities"."trackable_id" = $1 AND ("activities"."owner_id" IS NOT NULL)  ORDER BY created_at desc LIMIT 50 OFFSET 0  [["trackable_id", 1021]]
      User Load (1.5ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 3]]
      Announcement Load (1.4ms)  SELECT  "announcements".* FROM "announcements" WHERE "announcements"."id" = $1 LIMIT 1  [["id", 1021]]
      Rendered public_activity/announcement/_create.html.erb (19.4ms)
      Rendered admin/halls/activities.html.erb within layouts/active_admin (27.4ms)
     ...
     ...

    Completed 200 OK in 681ms (Views: 368.8ms | ActiveRecord: 56.4ms | Elasticsearch: 0.0ms)

Can anyone help me to figure out what is going wrong here.

Thanks in advance.