Filler materials within carts (Fluval, Polyfil, etc...) in my experience may burn in many circumstances. Most common causes are
eliquid not wicking fully from filler material to the atomizer (Causing the atomizer to cook the filler instead of vaporizing the
eliquid), dry cartridge, or especially with Manual Battery model of vaporizers extended puffs (activating the device for more than 5 seconds or longer depending on the model).
I had similar issues and switched to direct dripping of the
eliquid on to the atomizer instead of using the cartridge. Drip tips found with many vendors listed within ECF makes this easier. With either the eGo or Riva models, typically 3-4 drops directly to the atomizer provides a decent yield of 8-15 good tasting puffs before requiring more drops.
I hope this provides some relief and ask if we can provide more help.
