The most common reason for declined transactions in the US is that your card has not been “switched” on for the USA. CHIP & PIN is not common in the US, they rely on the Magnetic Strip on the back of the card. Unfortunately, just authorising your card transactions based on the Magnetic Strip on the back of the card is less secure than CHIP & PIN therefore your card is not automatically activated for a Mag stripe transaction.
The Mag-stripe on the back of your credit card and debit card (including Glints) used to be used to identify and authorise car transactions prior to the more secure CHIP & PIN. The Magnetic Stripe on you card contains basic information required to identify your card for a transaction and is easily copied or “cloned”. The CHIP is a more secure way of storing the card data and also has the added secondary external validation of requiring you to enter a PIN.
Some countries like the USA have been slow to adopt the more secure CHIP & PIN technology and still rely on Mag-stripe technology