Make sure your modem model is setup correctly with Comcast. I have a 6121 and it was setup in Comcast's inventory as a 5100. Worked OK, channels bonded, but no new firmware. Modem had to be removed from inventory and be re-provisioned the next day (devices clear out of inventory after midnight). Got the new F/W at provisioning. Log into your account at Comcast.com / My Services to check your equipment inventory.
This is what worked for me, hopefully it will help others.