I was once "Wensel Woodsman Sharpening Challenged" too. However, after following Charlie's how-to exactly step by step, they have become one of my favorite heads. While they may not get as sharp as a 2 blade head, it's pretty darn close...and with just the file I can get the edge on the WW simply wicked. And it is probably the simplest to sharpen once you get the hang of it.
Magic marker the edge bevel to make sure you are getting the surface cut level. Yes, there is some warpage in the blade during the welding process but the file work will even it out perfectly. In my experience, one of the keys is to take the same strokes each side....not working down one side at a time. If you get two perfectly flat and one still has dips etc., keep rotating working all three sides equally. One or two strokes at most per side before rotating.
Even pressure is another key, it's easier to put too much pressure on the back of the blade as you drag it rearward...you have to work at getting the feel of keeping the pressure even.
Progressivly lighter strokes...Once the surfaces are all bright and shiny with all magic marker gone, each blade rotation I make lighter and lighter strokes. Until I am just pushing the weight of the head, rotating each stroke, for about 10 strokes per side for the finish. If you want, you can run it across some other stone or croc stick or whatever, but you won't believe how darn sharp they will get just pulling them across a file.
If you still can't get it with a file, try a belt sander. Terry does his this way and they come out as good as any method I've seen.
Still having problems, send me one you're having trouble with and I'll video what I do to get that particular one sharp. Truly, these are one of the easiest heads to sharpen there is.