This happens when editing the post. In some wordpress installations, when switching from the text mode to the preview, the editor removes the script tags. This may also happen if the editor of the post is not an administrator but a less-privileged account (script tags need to be added by an administrator and editing the post with a less privileged account will remove them). 

What you can do is either edit the posts as an administrator and keep an eye on the codes or, even better, install the wordpress inlinkz plugin : http://downloads.wordpress.org/plugin/inlinkz-scripter.zip

You can search just for inlinkz inside wordpress and it will show up.

Then you just have to use the shortcode provided just underneath in the script page.

The shortcode does not disappear and it does not require administrative privileges to put anywhere in the post.