I know some WSMs tend to run either hot or cold. My 18.5" tends to run cold and I also sometimes have trouble getting temps up; other people on here talk about using 20ish briquettes for the minion method, but if I don't use a full chimney I'll have issues getting above 200* until a few hours into the cook. Maybe you also have one that runs a bit cold.
I also have no experience with ATCs, but putting a skewer under the lid is a good idea. Also, try flipping the access door upside down and then putting some kind of stick or something beneath it to hold it in place and then prop it open a little bit to get some more airflow. I've tried that a few times and the hardest part was finding something the correct length to prop that door open. It's been a few years since I've done it, but IIRC I just found a small forked stick from my yard and used that. Of course, that stick is now long gone, but even something that simple can work.
Best of luck to you on figuring this out!