If it stopped during the cook, that's normal. When you were done with your cook, did you close all vents and did the fire go out (unburnt charcoal left)? If so, that would show that the WSM is air-tight enough not to worry about the lid.
Also, Kevin's right that some gunk will help seal things up, but don't let that gunk build up too much. I actually had to take a screwdriver to my lid to pry it off once. Keep the lid rim and the middle section lip clean with some vinegar on a paper towel. I don't scrub mine, but I keep the build up to a minimum. A wipe every couple of cooks is sufficient.