Every moving part must seal to create the vacuum
Coil in tight
Top and bottom tight to the glass
No debris on the o-rings
O-rings on tank and coil in good shape
Airflow holes closed before you open the tank to fill
Do not overfill leave a gap at the top to assist with vacuum in the tank
Don't subject the tank to heat sources that can thin out the juice and force it to flood the coil. Heat can be closed car, outside temps, coil getting to hot from chain vaping, or too high wattage.
Wattage too low which allows flooding by not vaporizing all the juice on the coil
Crack in the tank
Burr on the the threads of any part
If you decide on a new tank avoid SMOK. I personally like the Vaporesso NRG tank or the Cascade. Also like the Aspire Revvo, top air flow it's almost leak proof but only holds 3.6ml. I just got my Freemax Mesh Pro tank 3 days, no leaks.
2 Links to videos on leaking tanks, one of which is a SMOK tank.
smok tank leaks - YouTube
leaking vape tank - YouTube