Try these steps: 1. Forget and re-pair the UE Boom in your iPhone's Bluetooth settings. 2. Ensure UE Boom is selected as the audio output in Control Center. 3. Update both iPhone and speaker firmware. 4. Reset the UE Boom by holding Volume Down and Power buttons for 10 seconds. 5. Check Spotify's 'Devices Available' and manually select the UE Boom if it appears.