PNGs from older FLA's don't compile in Air

I have some older Air apps I'm updating and I'm getting a graphic bug. How my process works is I create a graphical asset ( image, button, animation or control skin ) in Flash CC, assign it a class name and publish a SWC. I then use the SWC as a graphical asset library in Flash Builder.

I've noticed recently that PNG's from my older projects display fine in Flash CC but when I compile the app the image data is transparent in the app. This is only PNG's not JPG's.

I also have the same issue with a modern Air app created only 7 months ago when I migrate from Mac to PC or back and forth ( I develop on both OS' ). I have to export the image and then re-import it. Again the image appears fine in Flash CC, it's only when I compile it into the App that the bitmap data seems to go transparent.

Running Flash CC and Air 15.