Agreed. Topping off a carto is absolutely necessary when it starts to get low.
Some juices seem to vape out quicker than others too, so with certain juices you may find yourself topping off more.
Once you get to a point where even a top-off doesn't cure the burning taste, there are two options: 1. Toss it in the trash or 2. Try and wash it out real good, dry it, and fill it again.
Even after a wash, you may not be able to rescue some cartos.