Hey there! @magickarle,
It sounds like you may need to re-authorize the Google Integration, to work with Smart Things under the Works with Google secton. The setting is located under Google Home Settings>Works with Google>Smart Things>Select Re-Link account. If you receive any error messages, clear the cache and attempt again.
Here is a troubleshooting article to assist with updating the Google Home Intergration: Set up and use Google Assistant or Amazon Alexa with SmartThings.
You can confirm that it was properly linked by checking Linked Services in your Smart Things Application. You also have the option to delete Google Account from Smart Things to setup again and can re-set up using this pathway (+) Add Device>Voice Assistant>Google>Enter Credentials.
As you mentioned, you can also try Syncing your services “Ok Google Sync Devices” is the command to accomplish that action.
I hope this message finds you well!