You must include at least one positive keyword with 3 characters or more
The search module is a little weird in Drupal, and can take some time to wrap your brain around.  If you start reading from node_search(), you will see a call to do_search() which is the function that links the search indexes to the things you are searching over.

If you read that function, you will see that there is a variable minimum_word_size, that defaults to 3.  If you need to be able to search on smaller words, you need to set this smaller.  I believe you can set this on 'admin/settings/search', but I don't have any Drupal 6 sites to check against.

Change this value, then wipe the search index and rebuild it.  These last two steps are most easily accomplished with Drush.