I had problems with my first corn snake not thriving, initially. I don't know if this will help, but I only had one hide on the warm side of my 10 gallon at first and my snake stayed in there all the time. She didn't grow much and had a bunch of regurge problems. I put more smaller hides in there-one on the "cool" end and one in the middle, just off the heating pad area. Maybe your snake is forcing itself to be too hot or cold because of lack of hides?
My experience with my first corn is that she will only go into the warm hide directly after a meal and spends the rest of her time between the two cooler ones. However, I have not had the feeding problems you are experiencing.
I, too, have a corn that will be a year old in July and he is around 21" long and about as wide as a Sharpie marker at his thickest point. But, if memory serves, when I purchased both of my snakes, they were much longer than 11". I think one was around 16" and the other about 18" or 19".
Good Luck.