Just to help here, I came across several 'issues' in R77.30 where once the logging stopped to a log server , it never restarted by itself.
Only way we found was to create a dummy object in the policy, add that as the log server, push policy, then put back the original log server object, push policy again.
This was the only foolproof method we found.
However If you want to configure gateways to send any locally collected logs to the SMS/MDS once the connection is back up, you need to go under Logs > Additional Logging on the gateway or cluster object and configure Forward Logs to Log Server and specify a time interval (midnight). This may be different in R80 but certainly worked in r77.30.