Check the location of the NFC sensor on your smartphone. Some phones do not have an NFC sensor or it is blocked by the phone's operating system (iPhone SE, iPhone 6+ and older versions).
Check the location of the sensor on your reader. NFC technology has a very small range, only a few mm. The NFC sensor must be held closer to the reader NFC sensor.
If the the phone is not recognized by the reader, move the phone a few inches away from the reader and then move it closer again.
It is important to move the phone closer and further away from the reader on multiple attempts. If you touch the surface of the reader with your phone, the phone will probably not be recognized!
If your phone has a protective cover, it may be too thick or you may have another item stored between the protective cover and the phone. Remove it.
When surrounded by metal, some smartphones have a problem with NFC tags recognition. It is not guaranteed that your smartphone will successfully connect to the reader via NFC.