The core issue seems to be that if you book using a future travel voucher, the ticket has to be re-issued manually as the voucher is e-ticket number from the previously booked flight, so presumably they are inundated with requests. I imagine they then get a lot of people calloign worried, when a few days before your flight an email comes through confirming your flight has been ticketed, but it looks like you've booked and paid for a new flight. The staff I have spoken to recently all seem a bit fedup with the way the vouchers work!