If you cannot compress a file, check the following:
Check the Compression attribute for the volume to make sure it is enabled. To apply the file compression option to an existing NSS volume: From iManager, click Storage > Volumes > Properties > Attributes, select Compression, then click OK.
Check to see if the Do Not Compress (dc) attribute for the file or directory is set.
Review other factors that affect compression, as detailed in Factors Affecting Compression.