DV8

I'm having difficulty exporting a simple design as a PNG file & keeping the transparency that I want. The output files has a white solid non-tranparent background. Why is this? I was expecting it to be transparent & show up as a greyed out chequerboard. But it doesn't, I assume I have to check a box somewhere in the process, but can't find one that's relevant. Could someone point me in the right direction please.