If you suspect that grooming of your ACS database is not taking place, run the following query to check whether the status indicates it is in a groom-able state.

Use OperationsManagerAC
SELECT *
FROM dtPartition
ORDER BY PartitionCloseTime

Data will be groomed when the Status field = 2.  If you are still running RTM version, you may need to manually set the status from 1 to 2.  This was an issue only in RTM release, and has been fixed in SP1.  You can use the following update query to change the status, and data will be groomed during the next interval.

Use OperationsManagerAC
UPDATE dtPartition
SET Status = 2
WHERE Status = 1