Chins are difficult to progress late into the LP. As you've noted, bodyweight goes up fairly substantially (most of the weight is in the ass and legs) and chins are very sensitive to an increase in bodyweight. i.e. The addition of 5 lbs of bodyweight is much more significant than the addition of 5 lbs to a barbell on a squat, bench, etc.
I don' require my trainees to achieve 15 reps before adding weight. If they can get about 8-10 good BW chins I'll start adding weight on alternating workouts. Usually this is 10-15 TOTAL reps (ex 4-5 x 2-3) with maybe 5-10 lbs of added weight. Progress is slow, but it always seems that BW chin strength starts to go up quick once weight is added.