I had the same problem last week with an eGo.
Check your contact plate at the end of your batt with a multimeter to make sure you're getting voltage. This will tell you if the battery's got a problem or if it's a contact issue. If you've got voltage, then the head on one of your
tanks has pushed the contact plate down into the battery so it can't make contact with shorter reach heads. There are vids on this, essentially use a paper clip or your multimeter leads to dig into the rubber ring around the contact plate and pry it back up higher so the tank contacts can make contact. Don't screw your tanks on so tight, just snug them up.
If there's no voltage, then there's likely an internal problem with the battery.