[Database Full Export ] EXP-00056, ORA-01555 snap too old error

 I have a table with two BLOB columns and 7,000 rows.
While exporting this table I am getting the following error
 EXP-00056: ORACLE error 1555 encountered
 ORA-01555: snapshot too old: rollback segment number with name "" too small ORA-22924: snapshot too old

no corrput data : metalink 787004.1, 253131.1, 452341.1

desc dev_resource_contents;
Name                                      Null?    Type
----------------------------------------- -------- ----------------------------
RESOURCE_ID                               NOT NULL VARCHAR2(256)
META                                               BLOB
CONTENT                                            BLOB

* sucessfully case

exp usera/pwa file=t1.dmp log=t1.log tables=dev_resource_contents direct=yexp usera/pwa file=o1.dmp log=o1.log  direct=y

* error case

exp system/manager file=full.dmp log=full.log direct=y