Send to back

If you are going to use a background element (a rectangle) to form a background color, it has to be at the very back of all page elements, especially all layers. It sounds as if maybe you have your rectangle grouped with something that remains behind it no matter how you change the stacking order within that 'group' of items.

The optimal way to do this is to create a new Layer and place the background object within it, then move that Layer (best named something like BG or Background to minimize confusion) to the very bottom of the stack in the Layers window. Once there, lock the layer so that you don't keep selecting or moving the background object.:

You can do this within a single Layer, as well, as long as the background object is free to be sent all the way to the back of all display objects, and once there, you can lock it individually to keep from moving or selecting it again.