Voucher cancelled (but still an amount available)?!

I was gifted a R750 voucher from my husband for Christmas 2019. In January I bought 3 items, total was about R550 (Order ***). I left some money on the voucher to use later. A few days later I saw a pair of boots I wanted. I tried to use the remaining amount on the voucher, but at Check Out I got a message "This voucher has been cancelled". I tried a few times, but no success. I then sent an email (25 Jan) to their service@zando address, got a Query number (***4). No one has rep**** to my query yet. It is now a few weeks later. I would really like an answer as to what happened to the rest of my voucher amount (I did type the voucher code in the email). Benice Erasmus