For newer iPhones, you can also tap your dot.device to the “notch” (from iPhones XS to 13) located on the front of your phone at the top of the screen:
Or, the “dynamic island” (iPhone 14) located toward the top of the screen:
Be sure to use a little motion to wake up the NFC sensor as it may be sleeping.
The sweet spot for the iPhone XS and XS Max is a little further to the right.
For a Google Pixel, you can turn on the NFC sensor by taking the following steps:
Open your phone's Settings app.
Tap Connected devices -> Connection preferences -> NFC.
Turn on Use NFC.
The placement of the NFC sensor on a Google Pixel varies based on the model. Since the dot.classics and dot.thins need to be placed right on top of the NFC sensor, it's important to understand just where the sensor is. Please review the following Google article on the Pixel phone hardware diagram to learn more about where the NFC sensor can be found on Pixels.