Which Wismec mod did you get, and what batteries specifically are you using? (Make and model batteries)
Your tank could have a coil short or the mod may have a faulty electrical connection to the tank. The mod's 510 center pin could have been pushed into the mod from over-tightening the tank onto the mod.
Do you own an ohm reader to measure coil resistance? It could help determine if the coil has a short (would read 0.0 ohm). You might replace the coil with a new one and see if that helps.
We generally don't recommend putting a clearomizer tank that uses factory made drop in coils on a mech (unregulated). Their coils tend to vary widely in ohm resistance and often have inconsistant overall quality. Clearomizers should be reserved for regulated mods.