Make sure that the NFC reader on your phone is active
iPhone: starting with the iPhone XR and XS models, NFC is always active, no additional action is required. On iPhone models 7 through X, swipe to "Control Center" and press the NFC button. This action should be repeated every time you need to read the item.
Android: make sure that NFC is enabled. Easy to find by searching in the settings or swipe from top to bottom, the NFC button should be active.