EXP-00056: ORACLE error 1555 encountered

 Dear Experts,

We have scheduled a job to export full database daily, but one of the table is getting failed with "ORA-01555: snapshot too old: rollback segment number  with name "" too small ORA-22924: snapshot too old"  error.

I have verified in database undo retention and size of the undo tablespace. Here are the statistics

exp command:

exp \'/ as sysdba\' file=/u06/full_export_PNAS1.dmp full=y STATISTICS=NONE direct=y log=$EXPORT_DIR/full_export_PNAS1.log

SQL> show parameter undo

NAME                                 TYPE                             VALUE

------------------------------------ -------------------------------- ------------------------------

undo_management                string                           AUTO

undo_retention                       integer                         10800

undo_tablespace                   string                            UNDOTBS1