I got this lodge skillet at Christmas from my parents, so I'm new to the game. But I've tried to take good care of this skillet so far. When I rub vegetable shortening over the skillet after I've dried it on the stove, there is this spot on the bottom that won't take the shortening. It just kind of beads up instead of coating the pan. Any ideas? Remember, I'm new! In case it makes a difference, even though the pan came "seasoned", I have stuck it in the oven on 450 for an hour with vegetable shortening on it three times. Thank you for the help!