I have got into the habit of always switching on the Bluetooth on the smartphone first and only then starting the headphones. The other way round, pairing only works very hesitantly for me.
To pair the headphones, simply keep the "Play" button pressed when you switch on the headphones. This takes about 2 seconds after the headphones have reported "Power on" and is confirmed with "Pairing". You should then be able to connect to the smartphone.