Ted here from Shopify, thanks for posting your query!
When you log into your admin and open up one of the orders you mentioned, can you check the Timeline of the order at the end of the page and let me know which payment gateway was used or how the payment was processed?
Did you issue one directly from the admin without someone purchasing the gift card first? If so, then the gift card would essentially be a credit note. If a person bought the gift card previously then you should have received funds for that card within a different order.
When real payment is used to pay for a product, you will receive those funds within a few days. You can place a test order at the checkout as mentioned here if you would like and more info regarding gift cards can be found here.
If there is anything else I can help you with, please let me know, I'm happy to help!