Did you add it to your system using the App? 

If you accidentally added it to a second system a factory reset of the missing one and install to your original system will help.

Hi @SPACES Summer Hill 

Welcome to the Sonos Community!

I can see that your missing Play:5 is a part of the same system as the rest, so although @Stanley_4 had a good idea there, in this case it’s not the solution to the issue.

I recommend you get in touch with our technical support team who have tools at their disposal that will allow them to give you advice specific to your Sonos system and what it reports.

I hope this helps.