A common complaint I hear from customers with aging or inferior mattresses is that they feel like they are rolling towards the center of the mattress. Sometimes this can also feel this way when you sleep with a partner, as their weight combined with their partners causes the bed to cave toward the center.
However, this can happen on a brand new mattress too, and here are some reasons why this can happen on any mattress, from a memory foam mattress to an organic, latex mattress, either all latex or latex and coil construction...