Unable to use cubic entity selector

When I use cubic entity selector([x=n,y=n,z=n,dx=n,dy=n,dz=n]),
sometimes,It does.
But when I use it several times,it doesn't work.
When I use it,I have to use all dx,dy,and dz.
and I can't set a position to use it.
can't set x,y,and z.i can only dx,dy,and dz.