Maybe a dumb question.
I have two ubuntu server running the same services, the same operation system version, the same hardware, equal configuration - equal, because one as primary, the second as secondary server (DDNS, DHCP, etc.). That's the only difference.
For example:
I see 10 packages can be upgraded, i do it on the first (primary server). Then i go to my secondary server to do the same... where are not 10 packages to be upgraded, they are kept back. Maybe one or two days later, i can upgrade the package on the secondary server too - no longer kept back.