Frank, have you visually reviewed the bits that did not sharpen well to see if the sharpening wasn't complete, or the grind angle off? Any other possible failure modes?
In other words, do you know what is or is not happening when your sharpening isn't working?
In the cases here where the larger bits did not sharpen, many times a review of the tip indicated that for one reason or another, the grind angle was off. Most times it was off severely enough that the cutting edge was literally lower than the following shoulder, meaning that the bit could never cut.
My experiences have led me to believe that one of the following two things occurs when there is a problem. One possibility is that the larger diameter bits do not have the same twist rate as smaller diameter bits do. Since your sharpening alignment is totally dependent on locating off the center rib, that means that any twist rate difference affects the cut angle and orientation.
The second possibility also hinges on twist rate, but in this case, I've noticed that some bits do have a different twist rate than others. Same problem as above.
Mike: 1997 SE w/ 183K, aka White Horse
, Gutted Airbox w/K&N, drilled 180* t-stat, tint all around, clear corners w/SilverStars, "PONTIAC" light up tail, PEMs, NGK TR55s