simple_captcha does not work in test environment

Hi,
    I have been using simple_captcha and it works fine and make myself
happy. When I moved my app to test environment, It does not throwing
message when the letters in the image does not match with the user
entered text and it works well in my development environment. Can
anyone tell what will be the problem or How should I overcome this?
And I am afraid of use it continuously for my production.

Thanks in advance...

-Sadeesh kumar.