My guess is that the positive 510 pin on the tank is most likely pushed in. IIRC, that pin is spring loaded on the Nautilus so it could be that it's gunked up and the spring isn't pushing out the pin as far as it should.
You'll have to push the pin out from the inside of the base and clean it out. But be careful because the srping and the pin may go flying. lol
Also, assuming it's an old tank, realize that the rubber insulator around the pin may become damaged when you remove the pin as it may be brittle. That makes your tank useless until you can find a suitable insulator/grommet.
DO NOT try to use the tank without the insulator.
I have these from ft before but never checked if they fit for a nautilus tank.
$2.16 Rubber Insulation Rings for eGo Batteries (50-Pack) 50-pack - grey at FastTech - Free Shipping