This is a common problem. Even if there is a voucher code with huge discount, people don’t really believe that it is a real deal. Here are something that you can do to your voucher code.
Set a short validity date. Create the urgency to use it ASAP
Print it a paper or design your email voucher code professionally
Make it looks exclusive. Instead of “xmaspromo”, try “xmas4fans2018”
Make your promo code less restrictive so that customers can pass it to their friends to “abuse” it