Hey @NickG@Amazon I'm having this same issue - I successfully link my skill via auth grant code account linking and am able to use the skill. But after the accessToken expires (2 hours), AWS/Alexa isn't automatically refreshing the token. Did you guys have another tutorial on how to trouble shoot this?
This had me stumped, even after trying what appeared to work for some (appending ?access_type=offline to the end of the authorization URI)
The key part of this, had I paid closer attention to what @tkromrych, is that you have to go back to your google settings page and remove your skill from the list of approved apps. I was only disabling the skill and reenabling it, causing it to get a new token. Apparently this doesn't cause the access_type to register.
Here's the post with the solution (for me at least):
Hey, I'm having a related issue. Interaction between access token and refresh token will work fine for my skill if a new session is called. But, if the access token expires while a session is running, a new access token won't be created automatically. Or do I need to handle this case manually?