resending completed emails removes custom header

Hello,

I manage a course booking site and use your plugin to include joining instructions, I also use Ewout Fernhout’s “WooCommerce PDF Invoices & Packing Slips” to resend completed order emails if a customer doesn’t receive them or needs them sent to a different email. However when I use the resend order email, the custom email header is not sent with it, can you think of anything that may be causing this?

Thanks in advance (: