This is kinda like saying "I can back squat 185 for 9, but I can't front squat 185x1. Why not?" They're different exercises with different leverages and different muscle recruitment. Keep doing what you're doing.
Nothing at all wrong with sticking with chin-ups and getting them up to 10+ reps, and then think about working on pull-ups. If it's really bothering you (which it shouldn't), there are a ton of pull-up articles right here. Pretty much any one of them can help, but they often put pull-ups into the forefront of training.
Since you're just getting on a roll with your current plan, I, again, suggest sticking with the chins for quite a while longer.