Hi, I am A. Stevens here to help with your question! Do you have another device that you can try to sync with the Bose Solo just to make sure that device itself is not faulty? It is just odd that you can connect to the speaker but not the adapter specfically
Very odd, something you've probably already tried but can you try to toggle the bluetooth on/off on the nexus?
Have you attempted to do a factory reset of the Nexus perhaps?
Can you try this: Settings -> Apps -> Scroll over to All -> Select Bluetooth Share -> Clear Cache
That's very strange that for some reason it will only connect to specific devices, the only 2 things you can do software wise you have already done at this point (factory reset and BT cache cleared). If you want to attempt once more I would try resetting the device again and then clearing the cache before attempting to connect any BT devices. If that still does not work I would have to suggest contacting Google/Nexus for a hardware repair/replacement
If you are within 1 year of purchase you are still covered under the manufacturer warranty