Layering in PowePoint is a little different to layering in Word or Publisher.

Elements are layered one on top of the other in the order in which they were created.

If you want to change the layering order, right-click on any of the objects and then choose 'Order' from the menu that appears.

If you end up with a number of layered objects that you would like to move as a group, drag your mouse in a rectangle that encloses all the objects, and when you release the mouse you'll have created a group you can drag.  Click anywhere outside the group to 'ungroup' it.