Make sure your masster slide has been set to
allow objects on slide to layer with master
You will find this option when are using the Appearance inspector (which is the second from the left) and have a master slide selected.
You will not be able to alter a master slide object, it's placement or it's position relative to it's 'level' with other objects. You cannot even select it unless you are on a master slide (and most are locked on the master slide as well so they are not accidentally moved)
What you
can do, however, is send other objects to the back when the slide has been set to allow objects to layer with the master slide.
Gerry