Nearly always this is the phone, not the card. Every Somebody.Cards card also has a QR code and a printed code, so you can get to the same place without NFC at all.

Try these in order:

Wake the phone and unlock it. A locked screen will not open an NFC tag. Tap the right spot, and hunt for it if you have to. On iPhone, start at the top of the back, near the camera. On most Androids, the middle of the back. If nothing happens, move the card slowly around the back of the phone — the reader is a small area and it is not in the same place on every model. Hold it still for two seconds. A quick brush past usually does not register. On iPhone, look for a notification banner at the top of the screen and tap it. iPhones do not open the link on their own — Android usually does. Take the phone out of a thick case, especially one with a metal plate or a card pocket. Metal between the phone and the card blocks the tag. Check NFC is switched on. iPhone XR and later have it always on. On Android it is in Settings, usually under Connected devices or Connections. If none of that works, use the QR code. Open the camera and point it at the code on the card. Some older phones have no NFC at all, and the QR code works on every phone.

Still nothing? There is also a short code printed on the card. Go to somebody.cards/c/ and type it in — see What is the code printed on my card?