I do a pack of bone in chicken breast each and every time I smoke.
I put them right under the heat shield.
I'm not sure you cooked them long enough. I have them in for around 3 hours, then with an instant read, I consistently get 180* right at the 3 hour mark, keeping a chamber temp of around 240*
Maybe the marinade was too long.
I never marinade chicken, cause the skin seems to lock out alot of the flavor. Won't hesitate with beef or pork, but rarely ever do I do chicken that way.
And watch those tips of the chicken breasts. They dry out the fastest.
Keep spraying too!
Statistical Anomalies render some statistics meaningless. +/- 3 standard deviations
Bandera, M-BGE, WSM, Gold Kettle, Kenmore Gasser, 36Qt Turkey Fryer, ECB, Dutch Oven