If you have a smart phone with a magnifying glass feature you can use that to get a pretty good look at the condition of tip of your decapping pin. Another option is an old 28mm lens for a 35mm SLR, they make an excellent 10 X magnifier if you look into the lens with the film side around an inch from your subject.

What you want is a tip that is a true Hemisphere, as in a round sphere cut exactly in half. I found that my Dillon pins did have a "dome" shape but it was sort of like a dome on a football stadium with a sharp edge all the way around the perimeter. It is that sharp edge that bites into the primer and causes that "sucking" you are having trouble with. I would suggest that you don't bother with squaring off the tip, simply start radiusing the tip into a true hemisphere.