Hi guys, I post this in hope is of help if anyone is having the problem of synchronization with moodle and course kit regarding grades, the problem we encounter was that the timestamp of the server was off by 30 min, this made our moodle to reject the post from course kit, you only have to update this and you are ready to go!
Here is the link to the google forum, it explains in better detail: https://support.google.com/edu/coursekit/thread/10788250?hl=en
I know it is already in google, but I share here because I looked here first and I hope it can help someone else
Regards