iMac 5k not playing flash videos in full screen

Hi I work in IT and I'm working with a client who is having issues playing flash videos in full screen.

They just bought the new iMac 5k running Yosemite and flash videos will play fine in their regular view, but if you expand it to full screen, the screen goes black and the audio keeps playing.

We have tried reinstalling flash.

We found this issue doesn't happen in Safari, but it does in google chrome. We would like it to work in both.

Is anyone else having this issue? I'm trying to figure out if the flaw is the new screen resolution, the google chrome browser, or the flash player.