You should verify following collection criteria:

• Collection age

  From the Vault Store Partition Properties dialog box, you should verify the setting that you configured for the Collect files older than field. This field is located on the Collections tab. Make sure that you specify the age appropriately.

• Minimum files in collection criteria

  You can override the default minimum file value by creating a registry key HKEY_LOCAL_MACHINE\SOFTWARE\KVS\Enterprise Vault\Storage\MinimumFilesInCollection. That is a DWORD value and the data should specify the minimum number of files that can be present in a collection file.