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!
"...Best Buy sucks. They suck it long, and they suck it hard." - JazzyBadger
Bandera, M-BGE, WSM, Gold Kettle, Kenmore Gasser, 36Qt Turkey Fryer, ECB, Dutch Oven