First, for the avoidance of doubt, it is necessary to say that, in this context, a 'compressed' file is not a file produced by a compression program (.zip, .rar, .7z or whatever), but a file compressed by the NTFS file system, usually on a drive that has had compression enabled. That could be any file at all. Likewise, 'encrypted' means 'encrypted by the file system'.
Do you have compression or encryption enabled on the drive in question? If you do, Eraser will not erase any files marked as compressed or encrypted by the File System. This is a safety feature; Eraser 5 could corrupt such files when erasing others. Joel is currently working on low-level routines for the NTFS file system, so it is possible that he may be able to remove the restriction in future versions of Eraser.
If compression is not enabled, folders can still be marked as compressed, and Eraser 6.0.7. will not erase even uncompressed files within those folders. This is a bug, which is fixed in the development builds. So, if you are not actually using a compressed or encrypted drive, it might be worth trying a nightly build. These are not, officially, stable versions, but the build I am currently using (2241) is working pretty well on my machines.