It is common to have issues with scanning your sensor if you are moving your iPhone around too much. The key is to bring the area of the phone with the NFC directly to the sensor and then hold it still. Try bringing the front selfie camera of the phone directly to the sensor. Lift your arm up and use the front camera or a mirror so that you can see the phone coming to the sensor. The NFC on the iPhone is at the very top center of the phone.
Here is a guided video to help you through this process.