hi
if you are using exchange 2007 or exchange 2010
then you may need to Rebuild the Full-Text Index Catalog
1.Start the Exchange Management Shell.
2.Remove the index directories that are associated with specified mailbox databases:its in the same location as you storage group
Run this command from powershell
ResetSearchIndex.ps1 [-force] <dbname> [<dbname>]
3.Remove the index directories that are associated with all mailbox databases:
ResetSearchIndex.ps1 [-force] -all
taken from
http://technet.microsoft.com/en-us/library/aa995966(EXCHG.80).aspx