Any cook in paticular? ribs, shoulder, brisket?.. The most simple, short answer I could give would be that if you put a pork shoulder roast or brisket on with a liberal amount of rub, ran them at 235-250 and left alone lid on for 90% of the cook, you should have a nice thick bark. Ribs, chicken, other stuff of course would/could be different, along w/ other things ( I cook them hotter). Things that i've personally found to be bark destroyers are continuous basting/spraying, and foiling. Foil is a VERY usefull tool but long story short for now (to keep post short), depends what your doing w/ it and how long your using it, and if you follow up by finishing your cook on open grates etc. I do sometimes use it w/ brisket lets say, but each brisket is so different, so often. I actually prefer to not use it myself, but if the fat content/marbeling or the quality is poor you would almost hurt yourself (I think anyway) by not using foil. Personally as just ONE example w/ ONE pork shoulder only, never a reason to foil, bast, open lid for anything unless im in a time crunch (in my opinion). If your focus is bark then that statement could be stated X3. If your doing/trying high heat cooks I'll let the experienced help, i've never achieved what I wanted bark-wise with it at all, operator error no doubt.
I cant find it but a long time ago Kruger had said along the lines of "Bark formation will not occur until enough moisture has left the surface of the meat for carmelizing" or something like that. Basicly stating not to cook things at too low of a temp. Which makes me want to try high heat again confusingly enough.... but like I said operator error no doubt. My mistake MUST be in the foiling and then firming, i don't know.