It may be caused by the following:
Too frequent Vaping
Vaping setting is improper
Didn't prime coil before vaping
lack of e-liquid in the core
Defect coils
Please try to inhale for several puffs without powering-on while using a new core or let the tank sits for about 15 mins.
Please prime the coil with a low wattage for the first several puffs.