My two cents & experiences....
I think my "spot" moves higher/lower as a gain & shed weight. As a medium sized person I never had that big O. As a larger person it works best with him on top, as a smaller person it's best with me on top.
My second observation is that bodies go through cycles. Every body has a different cycle but they can range from every few months to years. You plateau while losing weight, your taste buds shed & rotate every couple of weeks, you have energetic bursts and moody times.... why not sexual rotations too? The more frustrated you get the harder it is. Relax, have fun, try something new