There are a few reasons that a tank could be leaking, and leakage is usually a sign of improper assembly. Make sure that none of the threads are crossed during tank assembly, and that all silicone gaskets are firmly in place. Also, check to make sure your O-rings have not been damaged even if you have a brand new tank. Make sure that the wick assembly is intact and in place. Make sure that the tank is not overfilled (only fill your tank about 2/3 of the way full), and that no liquid has gotten into the center cylinder. Also, please verify that the atomizer/coil has been properly screwed into your tank. If the coil is not properly in place, it may not fire, and it may cause juice to leak from the top or bottom of the tank.
TOPFILL TANKS: If you have a tank that utilizes the ability to fill the chamber from the top, it is advised that you close your airflow control while filling the tank, and open it to your desired setting again once the top is assembled. If you do not do this, it can cause a pressure differential in your tank that will force the juice through the bottom, giving it the appearance of leaking.
Additionally, tanks that have very old atomizers/coils are also very liable to leak due to the lack of absorbency from the build up of caramelized material in the cotton, therefore causing it to leak. In this case, you would need to change the coil.
The last thing to check is the thickness or viscosity of your juice. This will be determined by your PG/VG blend, and the nicotine level you are using. If your juice has a high PG ratio, this may be too thin to use in some sub-ohm tank systems, as it becomes even less viscous as it heats up. If you have a high nicotine strength, the overall liquid will be thinner than that of a low nicotine strength, which also contributes to leaking issues associated with thin liquids.