First you should turn on backup optimization.

Code:
CONFIGURE BACKUP OPTIMIZATION ON;
You may want to change this parameter as well.

Code:
CONFIGURE DEVICE TYPE DISK PARALLELISM 4 BACKUP TYPE TO BACKUPSET;
Then you should check to see how much of the flash recovery area that you are using. You may need to increase the size of this parameter.

Code:
col name            format a40
col SPACE_AVAILABLE format a30
col SPACE_LIMIT     format a20
select
name,
to_char(space_limit, '999,999,999,999') as space_limit,
to_char(space_limit - space_used + space_reclaimable,
'999,999,999,999') as space_available,
round((space_used - space_reclaimable)/space_limit * 100, 1) as pct_full
from
v$recovery_file_dest;