USE EnterpriseVaultDirectory
SELECT ArchiveName, FailedVolumeReason,
CASE
WHEN FailedVolumeReason = 1 THEN 'Directory Missing'
WHEN FailedVolumeReason = 2 THEN 'Too Many Consecutive Failed Items'
WHEN FailedVolumeReason = 3 THEN 'Sequence Number Mismatch'
WHEN FailedVolumeReason = 4 THEN 'Partially Deleted'
WHEN FailedVolumeReason = 5 THEN 'Data Missing '
WHEN FailedVolumeReason = 6 THEN 'Data Empty'
WHEN FailedVolumeReason = 7 THEN 'Data Invalid'
WHEN FailedVolumeReason = 8 THEN 'Volume Failed Open'
WHEN FailedVolumeReason = 9 THEN 'Volume Failed Checksum'
WHEN FailedVolumeReason = 10 THEN 'Volume Failed Stable'
WHEN FailedVolumeReason = 11 THEN 'Volume Failed Stable DA'
WHEN FailedVolumeReason = 12 THEN 'Volume Failed Compaction'
WHEN FailedVolumeReason = 13 THEN 'Indexing Engine Error'
WHEN FailedVolumeReason = 14 THEN 'Failed To Audit Items'
WHEN FailedVolumeReason = 15 THEN 'Duplicated Items'
WHEN FailedVolumeReason = 16 THEN 'State Missing Or Invalid'
WHEN FailedVolumeReason = 17 THEN 'Index Engine Metadata Sync Failed'
WHEN FailedVolumeReason = 18 THEN 'Incorrect Items'
END AS 'Reason Detail'
FROM IndexVolumeView
WHERE FailedVolumeReason != 0

Enterprise Vault (EV) can have problems with indexing specific items. In some instances this can cause an archives index to be placed in a status of failed. When this occurs a value is written to the IndexVolume SQL table in the FailedVolumeReason column. Below is a list of all of the potential values for FailedVolumeReason and their meanings. Additionally a SQL query has been written which will return all archives that are failed and the reason why:

Directory Missing = 1 - The index volume folder could not be found on disk.
Too Many Consecutive Failed Items = 2 - Too many items failed in a consecutive manner. Default is 25.
Sequence Number Mismatch = 3 - An Index Sequence Number (ISN) mismatch between the index and storage.
Partially Deleted = 4 - The Index volume has been partially deleted.
Data Missing = 5 - The Dataset file is missing from the index volume. 32bit indexes only.
Data Empty = 6 - The Dataset file is empty in the index volume. 32bit indexes only.
Data Invalid = 7 - The Dataset file is invalid in the index volume. 32bit indexes only.
Volume Failed Open = 8 - Unable to open the index volume on disk.
Volume Failed Checksum = 9 - The checksum file is invalid. 32bit indexes only.
Volume Failed Stable = 10 - An error occurred whilst saving indexing data to disk.
Volume Failed Stable DA = 11 - An error occurred whilst saving dynamic attribute indexing data to disk.
Volume Failed Compaction = 12 - The index compaction has failed.
Indexing Engine Error = 13 - There is a generic indexing engine error.
Failed To Audit Items = 14 - There is a failure to audit indexed items.
Duplicated Items = 15 - There are duplicate items in the index.
State Missing Or Invalid = 16 - The state.dat file is missing from the index volume.
Index Engine Metadata Sync Failed = 17 - An error accessing or updating the indexing engine metadata during a synchronization.
Incorrect Items = 18 - There are items with incorrect information in the index.

Please see the related articles document link for suggestions on how to resolve the issues listed above.