Just curious, how are you filling them? I'm fairly new to this, and when I started I ALWAYS filled from the top, and often had an issue whenI would start vaping on it before the juice had soaked into the bottom portion fully where the coils reside. Started filling them with the condom method, which shoots the juice in from the bottom through the hole in the bottom of the carto, and no problems since.
I still do get some brown stuff that comes out occasionally when topping off with my favorite, mostly clear Triple Rainbow from the Vaporcast store. That is just caramelization. That's why some people don't care for the clear cartos. They start looking pretty cruddy after a short time.