I have a question on smoking ribs. I use dry rub and apply this at least 24 hours prior to smoking them. I have a Smoker with a offset firebox and use a varity of hard wood, I smoke them for about 4 hours. (I remove the membrane on the back as well prior to smoking), They always come out tasting great but I've never been able to get the meat to just fall off the bone. Am I missing a step in the process?