I have been using spamassassin since a decade now. Few days a ago, sa was tagging a spam as ham. After few checks, I discovered that the sender address was white listed.
In this case, just use spamassassin --remove-addr-from-whitelist spamSender to remove the address from the white list. More information on this topic can be found on spamassassin's wiki page and the tool call sa-awl can be used to maintain the white liste database.