1. Coils don't last forever, and as a coil gets used, it's performance will drop and that can be one of the symptoms.
2. When replacing a coil, make sure to COMPLETELY saturate the new coil with liquid before assembling and filling the tank.
3. With a new coil, turn the wattage down for the first several puffs to make sure it is wicking properly before tuning it back up.
4. If this is happening with a brand new, properly saturated coil, remove the coil and use something small and sharp to gently rake at the wicking to try and thin it out JUST a LITTLE, then retry it.