Homepod is on same wifi network as my other devices but doesn't show up for airplay or personal requests
Went to Apple today This is a fix it worked for me 


The phone you use needs to be on the same wifi frequency 2.4Ghz or 5Ghz 
Set the frequency in the router settings to restrict devices to only one frequency not set to dynamic which allows alternation depending on circumstances 
you can check which devices are connected to the frequency's in connected devices in router settings 


I found that my devices such as phones connected to 2.4G when a few rooms away form my router but connected to 5G when close to the room with the router 

because my Home pods stay in the same place they connected to the same WIFI frequency's all the time 

if your home pod is on 5G  and then your phone switches to 2.4G the Home pod does not work as predicted 

KC