It could be a problem with the iStick or the tank, but it's probably not terminal.
First thing to check is the 510 connector on the Subtank. Make sure it is clean and there is no juice or grime buildup in the connection. Make sure the center pin on the Subtank base isn't stuck or loose. It's a floating pin and it could either get jammed up inside the base or become too loose and wobble around. Press on it with a toothpick or something similar. Make sure it moves and returns to it's original position, and is clean. If it's stuck you can probably get your fingernail under it and pull it out slightly.
Second thing to check is the 510 connector on the iStick. Make sure it's also clean and there is no juice or grime inside the connector. Use a paper towel or Q-Tip to clean it out and soak up any liquids that may be in there. The center pin on the iStick is spring loaded. Press on it with a toothpick or the Q-Tip and make sure it compresses down and then springs back up like it should. If it's stuck down you may need to get some tweezers and gently pull it up a touch.
If the problem is still happening after that then you've probably got a problem with the mod, but usually things like this are a 510 connection problem. Either dirty or pins being stuck.