UI Mask only working in Scene window.

I have a Canvas object with a Panel that has a Mask on it. I added an image to the panel that is twice the size of the panel. In the ‘Scene’ window, the mask works as expected. However, in the ‘Game’ window the mask is completely ignored. This is for a PC build. Is there some setting I am missing? I have tried it with an image and a raw image component with the same result.

I had a similar issue and sorted it with this:
http://forum.unity3d.com/threads/ugui-masking-blocked-by-background-meshes.270122/

Think it is something to do with masked UI not getting drawn if there is geometry behind it, looks like it will get fixed in 5.2, see: