Auto-increment problem

I have table defined as:

CREATE TABLE  messages (
	_rowid TIMESTAMP(14) NOT NULL,
	msg_mode enum('p','a') default NULL,
	acct INT(10) UNSIGNED NOT NULL DEFAULT 1,
	threadID INT(5) UNSIGNED NOT NULL AUTO_INCREMENT,
	re_threadID INT(5) UNSIGNED DEFAULT NULL,
	k_0 TINYTEXT,
	k_1 TINYTEXT,
	k_2 TINYTEXT,
	k_3 TINYTEXT,
	k_4 TINYTEXT,
	k_5 TINYTEXT,
	k_6 TINYTEXT,
	k_7 TINYTEXT,
	k_8 TINYTEXT,
	k_9 TINYTEXT,
	comment_remote TINYTEXT,
	comment_author TINYTEXT,
	comment_headline TINYTEXT,
	comment_body TEXT,
	PRIMARY KEY (_rowid, acct, threadID)
) TYPE=MyISAM

$sth = $dbh ->prepare("INSERT INTO $m_table VALUES (NOW(), ... $ acct, 'mysql_insert_id()'

does not increment threadID value, it's just 1 (in every row added). What gives. Ideas appreciated.

Romaneth.