My suspicion is that you're spending too much time on the tip...this grinds away rapidly and can begin to undercut the wing, which ends up causing that concave-to-convex shape that your photo shows. Perhaps more importantly.....slow down and grind the wings back a bit, but a little at a time. It looks like you've been a bit over enthusiastic and you're bluing the steel. If that's old, high carbon steel you might well be causing it to lose temper and soften, if it's HSS, you might be okay!
So....forget the tip, work the wings back until you grind back beyond the "S" shape.
Les