I've had that problem in the past and what I had to do was completely remove the UE boom from devices and start the pairing from scratch. Keep in mind that the UE boom can only connect up to two devices at once.
I can't find the links I've used in the past to get it working, so here's another one I found on Google
http://www.ultimateears.com/en-us/boom/support.
Under the question: How do I use UE boom with a computer?
Turn the speaker on.
Hold down the Bluetooth button (above the power button) until you hear a tone.
<-this is important to do or else it won't properly pair
NOTE: The Bluetooth connection button will blink rapidly, meaning the speaker is ready for pairing.
<-should be the top power button LED
Open your Control Panel.
Select Hardware and Sound.
Select Devices and Printers.
Select Bluetooth Devices.
Click Add a device.
Select UE BOOM from the list of devices, and then click next.
NOTE: If the Bluetooth Device Control window opens, expand the Headset type menu and select "My hands-free device UE BOOM does not have a display" and then click Apply.
Once your speaker is paired and connected, you’ll have to select the UE BOOM as the default playback device in the Windows Sound settings. Here’s how:
<-driver installation might take a while so be patient
Open your Control Panel.
Select Hardware and Sound.
Select Manage audio devices (under Sound).
Select "Speakers Bluetooth Hands-free Audio" and click Set Default.
Click OK.