How long does it take to smoke a chicken?
While cooking low and slow has its advantages, 6 hours is a bit too long. You should be able to complete a 6 pound chicken in 1 1/2 - 2 hours @ 275°F [135°C]. 

Some things to remember that might help you out:


You are going for an internal temp of 160°F [70°C] in the breast
meat and 170°F [75°C] at the thigh.
Every time you open the smoker you are allowing an ENORMOUS amount
of heat to escape, this prolongs the cooking time. (If I were to
bet, it would be that you are checking it too often)
Almost all the flavor your going to achieve from smoking is reached
in the first 30 minutes. You may want to smoke it for 30minute - 1
hour and then transfer it to a 275°F oven if you are having issues
maintaining the heat in your smoker.