I have managed to find what the problems is. The problem lies in the order my urls's are defined.
My app has below urls defined in the urlpatterns
url(r'category/(?P[-\w]+)/$', 'individual_item_page', name='individual_item_page''),
and if I put the above url before the zinnia url
url(r'^blog/', include('zinnia.urls')),
then when i click on "Add Category" (admin/zinnia/category/add) on zinnia category list in admin, it is going to the view individual_item_page instead of zinnia one's. Not sure why it is calling my app view rather than zinnia views.
I changed the order of urls by putting zinnia urls before my custom urls and it works fine.