Eraser almost certainly did not 'fail', in the sense that, if it completed the task you set it, that task will have been completed as specified. I would need to know a lot more about the particular sequence of events in your case to be more specific, but Eraser can only work on (1) file data that is actually available for deletion or (2) space that Windows has actually marked as free. There may be other locations, not directly accessible to Eraser, in which sensitive data resides.
It is now well documented, on this forum and elsewhere, that Windows keeps copies of deleted (whole or part) files in Restore Points and other protected locations within the file system. Any half decent file recovery program (I use Recuva) 'knows' about these locations and can find data in them. These locations can be dealt with in various ways (I use a combination of Eraser, CCleaner, and Recuva, which also has an erasing function), but you need to do this explicitly, and it seems from your description that you did not do so.
David