Check the following.
- Remove the headset from the charging case and make sure that the headset is turned on.
NOTE: You cannot make a one-touch connection (NFC) when the headset is inside the charging case because the headset is off.
- Keep the smartphone close to the charging case until the smartphone reacts. If you fail to connect, move the smartphone slowly on the N-Mark of the charging case.
- Check that the NFC function of the smartphone is set to on.
- If the smartphone is in a case, remove it.
- NFC reception sensitivity varies depending on the device. If you repeatedly fail to connect the headset with a smartphone by one touch, connect it to the headset by on-screen operation on the smartphone.
For details of how to connect with NFC function refer to the following Q&A:
How do I use the NFC function on my WF-1000X headset?