Your voucher will only arrive once you have fully paid your balance and payment has been taken. If you still havent recieved your voucher try the following things.
1 - Check your junk mail box.
2 - Log into 'Manage my Booking' on the website and follow "Re-send travel documents", you will be asked for your itinerary number and then you can chose the documents you require. If you are still facing problems after trying this, click contact us in my account, fill in the required form and someone will call you to resolve the issue.