I have created a couple of devices using the https://github.com/actions-on-google/smart-home-java sample project. I have also successfully linked my account and see these devices in my Google Home app. I am also able to click on a "On" or "Off" button next to the Lamp device, and I see the value change in Firestore.
However, when I click on the device it says "Not responding" on the top and "Offline" in the middle of the screen. I do not see any errors in the "smart-home-java" server logs.
Has anyone else experienced something similar? How should I approach debugging this?
As mentioned briefly in the comments, this is likely because your project is failing to successfully call Report State. The most likely cause of this is that your Home Graph API credentials are missing or incorrect.
In the current sample code, an error in this method call will cause the handler to return deviceOffline from a QUERY. You can also comment out or remove this line to debug the issue.