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 [link|https://livinginnovations.ph/minotti/|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.