When a new Sequencer becomes Active it makes sure the Recovery Log is fully in sync with the previous Active Sequencer for managed routing. If the Recovery Log is not in sync, the Sequencer determines the last in sync checkpoint in the two Recovery Logs and copies all data required, bringing the two Recovery Logs in sync before it becomes active.
If the log files are significantly out of sync, for example if a network outage between the Sequencers existed for a long time before the Active Sequencer went down and you issued the SEQUENCER TAKEOVER command, then the new Sequencer may need to copy a large amount of data. You may need to change the maximum amount of data the Sequencer copies with the parameter RecoveryLogSyncBackfill.
If the newly Active Sequencer is not able to contact the previously Active Sequencer to determine if the Recovery Log is in sync, then it will not become Active. This protects against possible data loss if some transactions had not been applied on both managed systems. If necessary, you may need to perform a takeover by force to allow the Sequencer to become Active.