EmailPW sending just [Session.NewPW]

Hi - I have at the top of my emailPW.php file this code:

<?php
if (!session_id()) session_start();
if(isset($_POST["Send_x"])) {
$_SESSION["NewPW"] = "".WA_RandomPassword(9, true, true, true, "") ."";
}?>

The page is generating the random password properly I think and it is being written to the DB.

However when the email behaviour sends out the new password, the recipient gets this:

Your new password is: [Session.NewPW]

The other fields (like username) are being generated fine. But the new password doesn't display - just the reference for it.

Any help would be really appreciated!

Thanks
Dan

More info on this - here is my code for the Email behaviour:

// WA_SecurityAssist_Email_1_SendMail

if($_SERVER["REQUEST_METHOD"] == "POST"){
//WA SecurityAssist Email object="Mail for Linux"
$WA_Auth_Parameter = array(
"connection" => $boomcms,
"database" => $database_boomcms,
"tableName" => "users",
"filterColumn" => "UserEmail",
"columnValue" => "".((isset($_POST["emailAddress"]))?$_POST["emailAddress"]:"") ."",
"columnType" => "text",
"usernameColumn" => "UserName",
"passwordColumn" => "UserPassword",
"selectColumns" => array(""),
"sessionVariables" => array(""),
"successRedirect" => "emailPW-thanks.php",
"failRedirect" => "emailPW.php?email=failed",
"keepQueryString" => FALSE,
"toAddressColumn" => "UserEmail",
"fromAddress" => "info@mydomain.com",
"fromAddressDisplay" => "mydomain.com",
"subject" => "Your password has been reset.",
"mailBody" => "Your password at mydomain.com has been reset. <br />Your new account details are as follows:<br />\nUser name: [UserName]<br />\nPassword: [Session.NewPW]<br />\nTo log in using your new password, please visit mydomain.com",
"emailFunction" => "WA_SecurityAssist_Email_1_SendMail"
);

WA_Auth_ForgotPassword($WA_Auth_Parameter);
}

change:
"sessionVariables" => array(""),

to:
"sessionVariables" => array("NewPW"),