The question really is, why do you get a bitter flavor when you smoke?
Normally it is from creosote buildup that has accumulated inside your smoker, which could be from poor ventilation inside your smoker.
What kind of smoker do you have, are you producing white, billowy smoke when you are smoking?
Please, give us more details so we may help. Is your smoker charcoal, wood, propane, pellet, electric? Vertical or horizontal? Kettle?