Have you ever had a gift card declined at a restaurant, even though you know it had enough money to cover the bill?
This could be due to something called "tip tolerance," where part of the gift card balance is held back for the gratuity.
On some credit card branded gift cards, there is a block in place if the card balance is not enough to cover the bill plus 20% for gratuity. This occurs in places where tipping is expected, like a restaurant.
If you have a $100 gift card, you might only be able to charge $80 on it for your bill at a restaurant. If the bill is $84.99, the card may get declined despite the fact that there is sufficient money on it.
For the most part, your server or the manager should be able to override this block if you say you will pay for remainder of the bill in cash or through another form of payment. If you are not allowed to split the payments, you may have to save your gift card for a different transaction.
Your card is not necessarily charged that additional 20%. That block is there to ensure that you have enough money to allow for a tip. If you only leave a 10% gratuity, you'll still get to keep the remaining balance, as long as you get the card back.
The same scenario happens at some gas stations, where the gas station may sometimes pre-approve $75 at the pump, even if you only spend $30 on gas. You get the rest of the money back, but the gas station wants to make sure it's there. If you do not have $75 on the card, you may need to go into the gas station and ask the cashier to charge an exact amount prior to pumping the gas.