You should verify following collection criteria:
From the Vault Store Partition Properties dialog box, you should verify the setting that you configured for the field. This field is located on the Collections tab. Make sure that you specify the age appropriately.
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.