Incremental replications rely on the existence of the snapshot taken during the previous replication. If this snapshot no longer exists, the data management engine performs a full replication. The data management engine automatically preserves the snapshots it needs for replication. However, there is an age limit applied to snapshots that are automatically taken by the NDMP system (including during a replication).
Snapshots older than the age limit are automatically purged from the system. The default limit is 7 days, but the limit can be configured through the NDMP History and Snapshots page. If the replication copy time is very long, or the interval between replications is long, then the default age limit must be extended.