Make sure that you remove any burrs with a coarse diamond stone before filing. The burrs become harder than the surrounding edge because of the heat generated at the instant of contact with the rock or whatever. They are often too hard deal with effectively using a file.
Also, use a quality file. Many hardware store files don't do well on skis nor are they uniformly flat. The steel edges used by many ski manufacturers are just too hard. A ski specific file like Swix or a quality file like Sandvik and such should do the job just fine. However, even the best files wear out.
You should only need to pull toward your body starting at the ski tip with smooth strokes for a good file to do it's job. See the "sticky" posted at the top of this forum for further pointers on edge tuning.