User information not getting if it is a same user

A checklist has many task_status, I have displayed the view in nested
model atttributes form,
checklist_instance attributes and its associated nested attributes are
getting updated correctly.
while i try to update user information to the checklist instance, the
following updates only if the updated_by_user is not same as

def update
@checklist_instance.attributes = params[:checklist_instance]
@checklist_instance.updated_by_user = @authenticated_user
        flash[:notice] = 'Checklist was successfully updated.'

Could anyone suggest me a solution for this..