Could be anything. Weak battery, flooding, cart not wicking. Gummed up resistor and so forth. I know this may be redundant but try a new cart.
For gumming up and flooding, put in a blank cart (no filling) and keep puffing until no smoke appears. This may fry the atomizer though :/ do at own risk.
Some people soak it in alcohol overnight then blow it out and let it dry.
Look in the tips and tricks section for more ideas.