I'm having trouble with an external Bluetooth speaker (a Bose Soundlink II, but I suspect it's not device-specific). After some delay, the device goes to sleep and turns itself off. This should be fine, normally, but it causes weird problems here. (I have similar problems when I turn off the device and then turn it back on directly.)
Linux (or, more precisely in my case, the blueman application) thinks it is connected to the speaker, but both the speaker and Pulseaudio disagree: the speaker is definitely not connected: the speaker stays silent when I play music and Pulseaudio doesn't see it in its list. When I try to disconnect and reconnect to the audio sink in
blueman-manager, I get this error:
Connection failed: blueman.bluez.errors.DBusFailedError: Host is down.
After another timeout, the speaker sees the host hasn't connected, and switched to the "blue" mode, in which it's waiting to pair with a device, at which point it can connect correctly, but that's a really long delay (about the time I took to write this question).
The workaround is to unpair the device and pair it again, then audio returns, but it's really annoying to have to do that every time the device goes to sleep.