I had an issue sharpening bits, for the most part it was due to not enough
time spent directly behind the cutting edge, even though I was heavily undercutting them.
Two things synergistic for me which made me get there.
Not holding the bit low enough on the bench grinder wheel, my grinder is on a low bench, and stooping lower more awkward.
Having a look at some bits, and copy... duh, you might be thinking?
Note some bits are ground with a smiley face shape to the flank/lands, and not tapering lands.
Getting that shape is more likely that you are removing material from directly behind the edge,
instead of it just appearing so, which large under cut bits disguising material what's left.
Even though this video is of sharpening using a large angle grinder, the movement is largely the same,
I watched as many engineer youtubers sharpen on bench grinders, but this one is one of, if not the best I've seen for some reason.
I guess its just better video angles or something, but it sticks in there better than others.
AvE,(that incognito gold miner, engineer/frankentooler youtuber feller)