some songs that play on my iPhone won't play on HomePods

• Make sure that your HomePod, iPhone, iPad, or iPod touch is updated to the latest version of iOS.
• On the iOS device that you use to set up HomePod, make sure that you are signed in to iCloud with your Apple ID and have turned on iCloud Keychain. You also need to have two-factor authentication set up for your Apple ID.
• Check that your iOS device has Bluetooth turned on and is connected to your home Wi-Fi network.*

Is this for Airplay?

Check your Airplay settings

1. In the Home app on your iOS or iPadOS device, tap .
2. Tap Allow Speaker & TV Access.- choose Everyone on this network

Play audio on HomePod from another device

• On iPhone, iPad, and iPod touch, use Control Center to select HomePod (and other AirPlay 2 speakers) as an audio destination. See the iPhone User Guide, iPod touch User Guide, or iPad User Guide for information about using Control Center.
• To play audio from other devices that support AirPlay 2, see the documentation for that device for details about selecting HomePod as an audio destination.

If it’s still not working you will need to be more clear on what “switching” means; Airplay or Handoff? Or are you asking Siri?

You might also want to reboot your wifi modem