There's a lot of answers here, but I don't think anyone said the magic words. To reseason cast iron, you need an oil high in poly-unsaturated fatty acids. Those are the kind that can chain together, and form a good polymer coating.

The thing that trips me up most about this subject is that 140 years ago, pork fat was very good for seasoning cast iron. Today, it isn't, because the composition of the fat has changed significantly.

The best seasoning coats will be thin, not appear or feel oily, give the pan a dark color slightly more glossy than an eggshell, and resist mild detergents, metal spatulas, and heat high enough to sear a steak on. If you have a layer of loose stuff in the pan, that's just a layer of gunk, and is probably adding some weird flavors to anything you cook.