How to place particleSystem between two ui

Unity3d 4.6.1

now, i hava place particleSystem between two ui;
like this:
canvas (sortOrder:1) whit GraphicRaycaster
A_ui
--a_button
----a_particleSystem
canvas (sortOrder:2) whit GraphicRaycaster
B_ui

But when I get the parent of a_button to B_ui, the a_button can't click, I don't know why?
like this:
canvas (sortOrder:1) whit GraphicRaycaster
A_ui
canvas (sortOrder:2) whit GraphicRaycaster
B_ui
--a_button
----a_particleSystem

who can help me ? thx