PayPal PDT works in sandbox but not live

[edubb]

I am trying to add a Paypal Payments button onto our website. I have Auto Return and Payment Data Transfer turned on.

When I point to sandbox, everything works correctly and it returns to my website with the transaction id in the url.

When I point to production PayPal, no transaction id is returned. Payment does go through.

Here is the form code:

    <form action="#VARIABLES.strHostAddress#" method="post" target="_top" id="testform">
        <input type="hidden" name="cmd" value="_donations">
        <input type="hidden" name="business" value="#VARIABLES.strBusinessEmail#">
        <input type="hidden" name="item_name" value="#VARIABLES.strGiftDesignation# - #VARIABLES.strGiftDesignation2#">
        <input type="hidden" name="amount" value="#VARIABLES.intPayAmt#">
        <input type="hidden" name="first_name" value="#VARIABLES.strFirstName#">
        <input type="hidden" name="last_name" value="#VARIABLES.strLastName#">
        <input type="hidden" name="address1" value="#VARIABLES.strLine1#">
        <input type="hidden" name="address2" value="#VARIABLES.strLine2#">
        <input type="hidden" name="city" value="#VARIABLES.strCity#">
        <input type="hidden" name="state" value="#VARIABLES.strState#">
        <input type="hidden" name="zip" value="#VARIABLES.strPostalCode#">
        <input type="hidden" name="email" value="#VARIABLES.strEmail#">
        <input type="hidden" name="cancel_return" value="#VARIABLES.strCancelPage#">
        <input type="hidden" name="return" value="#VARIABLES.strThankYouPage#">
        <input type="hidden" name="rm" value="2">
    </form>

where #VARIABLES.strHostAddress# is "https://www.paypal.com/cgi-bin/webscr" for live or "https://www.sandbox.paypal.com/cgi-bin/webscr" for sandbox.