In each ION Meter, the Diagnostic module has a "Time Sync Count" output register. This register indicates how many time-synchronization signals have been received. The value increases with each signal received. In some cases, this register is not increasing, although it is obvious the meter has been re-synchronized.

This is most likely because the synchronization type is set to NTP (Network Time Protocol). Using the ION synchronization, ION Enterprise detects that the meter if off-sync, and will send a sync signal.
When using NTP, the meter ignores the sync signals coming from ION Enterprise. With this setting, the meter doesn't actually RECEIVE a synchronization signal, as it will synchronize by itself on a network clock. That's why the time sync counter doesn't increase when NTP is selected.