Multi-zone group plays in only one zone

I have a system with six zones. It has worked for several years. Individual zones still work fin. I can play different music in different zones simultaneously.

Recently (last few days), when I group more than one zone together, music only plays out of one of the zones in the group at any given time. The other zones remain silent.

Is there a setting that is messed up in my system? Or is there something that can be reset to kick the system and make it work? I tried power-cycling a couple components, and also power-cycling my wireless router, to no avail.