¿No te muestra ningun tipo de error?
Asegurate de que la palabra que estes buscando no sea muy comun. MATCH AGAINST no te muestra los resultados que aparecen mas de 50% de las veces.
Cita: A natural language search interprets the search string as a phrase in natural human language (a phrase in free text). There are no special operators. The stopword list applies. In addition, words that are present in 50% or more of the rows are considered common and do not match. Full-text searches are natural language searches if the IN NATURAL LANGUAGE MODE modifier is given or if no modifier is given.
Tambien revisa:
http://dev.mysql.com/doc/refman/5.1/...stopwords.html