PNG Transparency Suddenly Not Working in Illustrator

Hi,

I'm suddenly having a problem exporting my images with a transparent background. When I export an asset it is appearing with a white background even though the transparency grid shows that the area should be transparent. I have never had this issue before and it just suddenly started, even on documents that I have been successfully exporting transparent images on for months. All of the sudden the same images and the same process I always followed is producing a solid background.

I did recently switch from a PC to a Mac and I am not sure if that would be part of the issue?? I don't see why it would but thinking about it, I don't recall checking this before I switched.

I have no idea how to fix it. Any suggestions?

Thanks,

Holly