There are a few steps to take if you are having issues connecting with a HomePod receiver.
Check HomeKit settings
Check to see if the HomePod is set to play notifications from your other devices. This is a HomeKit setting that is often enabled automatically when linking the device. You can find some more details about this setting in the link below: https://support.apple.com/en-us/HT208939
Ensuring the Allow Everyone setting is enabled for the HomePod
You will need to ensure that your HomePod is set to Allow Everyone on the Same Network to connect. Some more details on this can be found here.
Try giving the HomePod a factory reset
There may be a corruption or error with the current firmware installation on the HomePod. Giving the HomePod a Factory Reset and not enrolling it in HomeKit can correct most connection issues with the HomePod. More information about how to perform the factory reset can be found here: Factory Reset HomePod
Ensure the HomePod is not set up in Stereo Mode
Launch the Home app on your iPhone or iPad.
Touch and hold the HomePod pair.
Swipe up and tap the cog icon in the bottom-right corner of the screen.
Tap Ungroup Accessories.