We have 3 groups (Alpha, Beta, Stable) associated to 3 channels (Alpha, Beta, Stable) respectively.
I created a new package today and updated the Stable channel to point to that. All the CoreOS instances in Stable group should ideally start updating to this new package.
BUT the update has not yet started. There are close to 500 instances in that group.

I tried the same for Alpha and Beta and the instances in those channel got updated successfully.
Here is the screenshot which shows the set of Activities for Alpha and Beta channel where once the channel starts pointing to a newer version, the next activity we see is Roll Out started for the corresponding group. Same is not happening for Stable channel.

Also, if you see this:

Updates Enabled flag is set to False for stable channel.
I tried editing this flag. It gets set to True, but gets switched back to False in less than a minute.
Although when I checked the API response, it mentioned Rollout in Progress as True.

It has been more than 4 hours now and I am unable to figure out why none of the instances has been updated yet.