This.In my experience if your getting a lot of carbon build up in your food its because you burnt your last dinner, typically cooking with too high heat. Always start with low heat, the pan wont warm up any faster but if the cooking oil is smoking its too hot and will burn the food and create sticking in the pan and carbon build up. A cast iron pan/pot will not stick food if used properly.
It happens and if you burn or stick food it takes a while to cook it out with normal use, that spot in the pan will continue to stick food for a few more times. You can scrub it relentlessly but you need to get the oil back into the iron pores by properly seasoning the pan, may take a few seasonings. Once its clean reseason the pan and cook with it until you get the non stick surface cast iron is famous for. Just keep the pan seasoned regularly and dont burn your food and it will last. Sometimes when I get a sticky spot I just know to use the other side of the pan for non stick cooking like flipping eggs etc until the other side comes back around. (Thats mostly cause my teen burns everything no matter how much I say to lower the heat..... lol)
What you used originally or subsequently to season your pan can really matter as well. Using a high smoke point oil prevents a lot of the issues with potential pan fouling due to overheating and such. Or rather, it's much more forgiving and requires less maintanence if you're still working up your seasoning. Observing what's okay to put in your pan for the first while, and what not to, can also help avoid headaches.