Having a problem with a room not showing up in the SONOS S1 app

Dining room Bridge? You are still using a Bridge?

https://support.sonos.com/en-gb/article/remove-a-bridge-from-your-sonos-s1-system

The Bridge is so long obsolete and my memory so rusty…

I don't think the Bridge or newer Boost show up as Rooms as neither are capable of playing audio.

I have a BOOST, and it does not show up as a ‘room’ in the Sonos system, as music can’t be ‘played’ by it. 

A Bridge still shows in the S1 app and you can rename it.

Interesting. It shows as a ‘room’, or a device in the list of devices? Mine shows as a ‘device’, in that list, but not as a ‘room’ that plays music. 

When I first read the initial post, I assumed they had a ‘Dining Room Bridge’ and a ‘Dining Room’ under the Settings tab.

Reading it again now, I think they have a Bridge and a Dining Room, and now seeing their profile a few other rooms!

The OP states it is the Dining Room that is missing from the rooms tab.

Now, my advice would be to check the Dining Room speakers are connected to power and try rebooting them.

The initial advice to remove the Bridge also still applies.

​@DCW - What speakers are in Dining Room? Also does their Status Light LED give any indication of their status?

Sonos 3 & a dim white light

Great,  keep an eye on the Bridge, particularly the power supply as it has partially failed for many folks making it hard to identify the issue.