The only time I close my stack damper is when I am finished smoking, to keep out the bugs and rain. I always leave it open while smoking,
So the stack damper is not the problem, you said your wood was seasoned, shouldn't be the problem, If you are maintaining a pretty constant temp and and air and smoke are moving through and out the stack, lets talk about smoking times, and wrapping. and how much smoke you are looking for. Most of the time I use lump charcoal and a a few splits of hickory, pecan or oak. On occasion I will just smoke with all wood, When I do my meat is still not over smoked or too smokey for my. I like a very mild smoke flavor, and pretty picky about to much smoke. Only other things I can think of is how far your stack extends down into the smoke chamber.
Gary S