Then you should have had plenty of smoke flavor. That, at least has been my experience. However, if the chunks were small and combusted, they could have burned up and exhausted the smoke rapidly. A larger denser chunk tends to lose combustion to to a lack of oxygen; the tightness of the smoker lets very little air in and combustion uses that up rather quickly. Set the chunks in the box so they are between the heating element loops., they may smolder longer. I typically get smoke for 2 1/2 to 3 hours, sometimes a tad longer. All, you might want to try a stronger flavored wood. Your bark should have had good flavor, but then again, maybe not.