The problem is that you're sharpening too far up the blade. I'm guessing that you start at the base of the blade, and go all the way *past* the tip on each stroke. That's the error. Because you're putting pressure on the edge, each time you sharpen past the tip, you're also filing the tip down. I used to do this all the time freehanding when I was learning to sharpen. This is where a bench stone comes in handy, because it's easier to do just the tip without rounding it off than it is with a thin tool like those on the Sharpmaker. Personally, I freehand with a round ceramic stick then finish the tip with either a small, flat ceramic stone or a green (9 micron ultra-fine) DMT diamond stone.
The object is to not go past the stone, as this will round the tip down.