There is a list of common words that don't get indexed (the idea being, I think, that they would make the search index big and not do a lot to help find what you're looking for). The search is based on the mysql database text search, so you can find a list of the "stop words" in the mysql docs:
http://dev.mysql.com/doc/refman/5.0/...stopwords.html
Let us know if it's not one of those words, maybe you have a different problem.