Are you left handed? This can sometimes affect the usability of a touchscreen, I've discovered. You sometimes notice pointers on touchscreens are located slightly to the left of the place you actually touch (this is handy for righties - so the tip of the stylus doesn't cover what you're touching), which poses trouble when you're writing with the stylus leaning leftwards.
If you're not left handed and calibrating the screen doesn't help, it may be one of three things:
A. Your DS is busted. If it never works right no matter what game or stylus you're using, this may be the case. Get a refund/replacement on your DS.
B. Your stylus may be slightly warped causing you to think you're touching with one part of it, when you're actually touching it with another. This would be weird and unlikely! Get a new stylus
C. The game you're playing is badly coded. This is nobody's fault, really, and can't be fixed.