I doubt it will help, but try lowering the time from 1hour to 5 minutes and remove any throttle.

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Then refresh your servers and they should try to replicate more often.

However, I suspect it has been stuck for a while so this may not help....

Check DiskSpace on your Primary.....

Maybe Restart Services.....(Clearly Done in your case due to reboot...)

The ZCC setting above may help to complete replication faster after resolving a root cause such as low disk space...

Make sure your Primary is at debug....and review Loader-Messages.log for any errors as well.....This is another place the increased replication frequency may help.....since any replication errors should re-appear in the logs frequently.

Thanks Craig, although I already at that option set at 5 minutes.  It took about three hours for the system update to get to that server...

I'll check out the logs.  

If it did eventually get there...I would not be too worried....Sometimes it can be slow.....I believe System Update Content has a lower priority than other content since often it is not needed as quickly as an update to a bundle/policy/etc.....