P: Auto Blend Layers not focus blending - Photoshop 2014.1

I have 4 registered images of fungi, all images are at the same exposure settings but focused at different levels within the frame. I need to 'blend' together to make one image with a combined DoF.
Ok, so I've imporetd into PSCC as 4 stacked Layers, non of which are Background Layers. I've selected all 4, auto alligned and then used Auto Blend Layers [ABL]. The outcome is that the top 3 layers have totaly black masks applied and the bottom all white. All I basically get as an output is one of the 4 images, no selectivity whatsoever, presumably because of the 3 totally black masks. It also executes the process very quickly.
I assume ABL should be selecting the better focused areas on each image [white on the black masks] and combining all to give me one image with collective DoF. This isn't happening