It's a limitation of the vBulletin board messaging software; the search engine and MySQL database only searches on words with 4-characters or more. It will also skip some common words, even if they are longer than 4 letters. There are
workarounds that the board administrators can implement, but by default, out of the box, vBulletin won't do it.
Hope that helps... it's not because of anything you're not doing correctly.
Workaround for YOU would be to use Google, and the "site:" keyword to narrow results to eccie.net. Doing this, you could, for example, find all public posts since the last time the site was crawled for "sex" and "dfw" and "cim" and "review" in the same thread. Note that you'll need to disable SafeSearch on Google to see all results.
Here's how to do it:
demo