Chromecast Audio Multi-room doesn't friggin' work

I'm having a hell of a time getting Chromecast Audio multi-room to function reliably. I can cast reliably to each individual CCA. But when I cast to a group, only one or none of the CCAs will work. It is also completely random which CCA will decide to play.

I've rebooted each CCA to no effect. Each has the latest firmware (1.22.78337). And as I said previously, I can cast to each individual CCA with no issues so I can't believe it's a network connectivity issue.

Help!

EDIT: I'm using a Belkin N+ router. Has anyone else had compatibility issues with CC / CCA and this model of router?