10GR2 Parallel DML Running in Serial, but why?

I've got many parallel DML statements from a vendor supplied procedure.  Each of them gives the correct parallel hints. The statements all look something like these examples:

DELETE /*+ parallel(a,8) */ FROM HSASYS.CLM_APPLC_DTL_ADT a
WHERE ( ROWID ) IN
( SELECT /*+ parallel(b,8) */ STRROWID
  FROM IDENTIFICATION.CLM_APPLC_DTL_ADT b
  WHERE b.lrunseq = 32);

DELETE /*+ parallel(a,8) */ FROM HSASYS.FIN_VOU_APPR_DTL a
WHERE ( ROWID ) IN
( SELECT /*+ parallel(b,8) */ STRROWID
  FROM IDENTIFICATION.FIN_VOU_APPR_DTL b
  WHERE b.lrunseq = 32);

For each statement only the name of the table changes.

The weird thing is that up to a point, the statements execute in parallel as expected.  But after a point, they all run serially.