Probes do content checking very well, and its an important measure on if a page is responding with content that you want and not some error page.  However, when you setup a probe to do content checking, it is important that that you follow these guidelines:

1. Only do content checking on a direct URL.  Content checks will not work on a redirect.

2. The content check will only check clear text.  It will not check for javascript.  Also, be sure that the text is actually on the page your want to test, and not in the header or footer.  You can use view source in your browser to see what text might be best.

3.  Keep it simple, if you are testing for content on a login page, maybe test for something like: XYZ Company Login.  

4. Content checks are character sensitive.

5. Try to avoid using special characters.