why observer can't fire in rspec? help .

I read the guide and api and i created Observer on model, she can run
on rails console of test env.
but when i use rspec , the observer can't run in, it's never step in.

some good man help me, please. i debug it use one day and one night, i
can't sleep in today night unless i solve it.

Does this help?

http://stackoverflow.com/questions/33048/how-would-you-test-observers-with-rspec-in-a-ruby-on-rails-application

very thank you.
i know how to test observer now.
but i can't understand why observer can't run in spec?