Pointer Enter event doesn't fire Unity 5.5

I have two images with “Pointer Enter” event triggers that share the same parent. When I click down inside one image and drag it into the second image, the second image fires the Pointer Enter event. When I drag from the second image to the first image, the first images Pointer Enter event is never fired. When I switch the order of the images in the hierarchy, I get the opposite results. The first image fires the Pointer Enter event but the second does not. Does anybody know why this might be?
Thanks in advance.

high depth image will cover low depth image, and low depth image
will can’t receive ray,if image can’t receive raycast,this image event will can’t work;