Check if you have set the hidden input field.
<input type="hidden" name="code" value="0-1396447326-etc">
The value is generated by q2a using:
<?php
$this->output('<input type="hidden" name="code" value="'.qa_get_form_security_code('answer-'.$question['postid']).'">');
?>
You need to specify the correct item for qa_get_form_security_code(). Check all files in qa_include folder for this function to get an overview.