Some settings to look at in Enterprise Manager->

Database – differential
Perform a differential database backup. A differential backup records only the changes made to the data in the database after the last full database backup.

Append to media
Append the backup to an existing media. The previous contents of the media remain intact, and the new backup is written after the end of the last backup on the media.

Overwrite existing media
Specify to overwrite any existing media. By overwriting backups on media, the existing contents of the backup media are overwritten with the new backup and therefore are no longer available.