It seems like the problem I faced when I was trying to make a 2d sidescroller...If you aren't opposed to using z coordinates,then try
also adding an alpha material..meaning this material to your sprites :-

material mat_alphatest
{
effect =
"
technique alpha_test
{
pass p0
{
zWriteEnable = true;
alphaTestEnable = true;
}
}
";
}

regards
zazang