Sometimes over tightening an atty/carto with a protruding center post will force down the insulator on the mod causing a short. Try removing the atty, wiggle the center post on the mod up a bit, then gently reattach the atty. Before getting it snug, check to see if it fires. If it does, then the atty center post is protruding too far. If not, give it a quarter turn and repeat until it does. If the atty center post sticks out too far, just shove it in a bit gently. That should fix your problem. If not, you may have a break in your insulator which will require a bit of work to fix.