Can't see all multiroom devices in web or app

I have 4 raspberry pi with Volumio installed. They all run latest Volumio version 2.599
I can access all the instances using browser with ip address. In the multiroom devices I can’t see all the devices.
The Volumio “room1” is reachable in http, I can mange well the instance playing songs ecc… if I go in multiroom devices I see other instances of Volumio. If i change instance, I can manage new Volumio instance but if I go in multiroom devices I can’t see room1 instance and go back to manage it.
When it happened, from other instaces I can’t see room1’s instance.
I have same problem also with andoid app that I purchased.
Sometimes the problem solves by rebooting Volumio or I have the problem with more than one instance so in multiroom devices I find only two Volumio’s instances. I also set one IP in the app setting but I have same behaviour (I suggest to let the user put all the Volumio’s ip addresses if he know them, non only one)
Sometimes reboot don’t work and I still can’t see Volumio instance.
I don’t have network problems, and raspberries are connected with ethernet cable,only one has wifi
Someone could help with this problem please

Hi massi, pardon my ignorance but do all your rpis have unique hostnames? Do all your rpis have static IP addresses?

Hi Crumpeldoor, all my rpi have a different ip with a dhcp reservation. I haven’t configured a DNS for hostnames, but I can access all of them with ip address in browser. I set a unique name in first volumio wizard configuration so hostname are different.

ok, what about being to able to ping each rpi when they are all turned on? do you get [timely] responses from each device?
If you are in MS-Windows do a 'ping -a [device IP address] to see what hostname is being returned

I update all the devices to 2.609 and now is working well. I do the ping and hostnames returned are different. No clue about the cause of the problem. I don’t change anything, maybe it was a bug, but I don’t see anything in changelog related to this problem…