Q: Macbook Air Screenshot Malfunction

Hi there,

So I can take screenshots on my Macbook Air by using 'Command + Shift + 4' and the pictures show up on my desktop. It works perfectly fine (most of the time).... EXCEPT when I'm watching a movie, and I want to take a screenshot of it while it's playing (on DVD Player), the picture still shows up on my desktop but it's either completely black or grey. Sometimes it works, sometimes it doesn't, mostly the latter. And I'm not sure what to do. I've also tried 'Command + Shift + 3', but it has the exact same result. Anyone please know how to help?

Thanks!