People are ticklish to different sorts of stimuli. Some are very sensitive to light touches (an ex a wrote about in the true story section), while others are more sensitive to slightly stronger touches.
Tickling is also not purely about how effective you are. You must also take into account your partner, your 'lee, and try to give them pleasure as well. I know a girl who is horribly ticklish. She couldn't possibly stand being tickled on her feet by fingers, let alone something as lethal as a hairbrush. However I have always wondered if a lighter tickle, such as a feather, might maybe even be enjoyable to her.