How to extend the timeframe for flashback queries ?

Hi all,

I want to extend the timeframe for flashback queries:

- Undo tablespace is fixed size.

- UNDO_MANAGEMENT parameter is set to AUTO

- select flashback_on from v$database -> NO

So I've increased (from 12 to 16 GB) the Undo tablespace size, but it remains more or less at 63% full, and this for several weeks now.

I have noticed an improvement in the timeframe, from 28 to 37 days (according to V$UNDOSTAT.TUNED_UNDORETENTION), but some queries in this timeframe, such as:

SELECT * FROM <a_table>

VERSIONS BETWEEN TIMESTAMP TO_TIMESTAMP('23.03.2015', 'DD.MM.YYYY') AND SYSTIMESTAMP

WHERE <a_condition>;

fails with an ORA-08180 error.