Different indexes cores could have different definitions. You can check $FWDIR/conf/log_policy.C for advanced options. There's a chance that the logs are available but indexes are not, you can check it by trying to open log file manually of older dates and see if you can see the event in this mode (which is non-index mode).
Also, you should check log retention definitions on the SmartEvent server itself. If the server is short on storage it might trigger emergency log cleanup and might explain why this isn't available.
Kind regards, Amir Senn