The edges of the flint need to be sharp, and angled right to machine bits off the steel. Think of the angle of a lathe tool, kind of flat topped, and undercut by 45-60 degrees (very roughly
) Some sources of steel are better than others. Files are very hard, but better yet they tend to be high in carbon and low in allow content. Fire steels made from spring steel don't seem to work quite as well, and there is some possibility that higher alloy content (tool and bearing steels) don't work as well either.
There is some technique to it, but if you have been practicing a bit you probably have enough technique to get it to work.
If you can get an old file with one edge free of teeth you can test the flint. Not sure how you would test the steel though.