Send them an email and ask them to escalate the issue to a higher tier and ask for replacement voucher codes or credit on your account for the lost vouchers saved.
This is happening to quite a few people with missing vouchers saved on their account after applying them from a NOW Stick or Box.
Use the email address on this screen shot below.
Your best to send an email to customer support. Hopefully the staff at NOW can sort it out for you.
Address in below picture.