9.2 - Parallel Query Not Working

On a 9.2.0.6 database on Solarix 64, which is created every day from a magical 
SAN-based mirror process thing (excuse the tech talk), we alter a particular 
table to a default degree of parallelsim of 12. A process which performs a full 
scan of that table picks up the degree of 12 and completes in a couple of hours.
   
  Later in the day we can no longer prompt the instance to give any 
parallelism. 
   
  I've tried everything obvious ... setting parallel_adaptive_multiuser to 
false, parallel max servers to 48, used hints, made sure the table's default 
dop is 12, checked that PROCESSES is reasonably high (500+), that the server is 
not very busy (less than 10 active sessions), ALTER SESSION FORCE PARALLEL 
QUERY, but nothing we do will prompt PQ to be used on even the most simple 
SELECT * FROM ... queries.
   
  Any thoughts on this are much appreciated.

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