Hi scubamom,
This is fairly normal with larger size mattresses because the center will soften and break in at a slower pace than the parts of the mattress you sleep on. Sleeping in the middle can even things out and if your bedframe makes it possible you can also rotate a king mattress 1/4 turn and this will help even things out as well (see post #2 here).
It’s also possible that it could be related to any flex in your foundation because a split twin XL foundation (which is used with a king size) will have less flex in the center where the rigid sides of the foundation meet than in the middle of each side.
Phoenix