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Are you referring to "Command - Shift - 4" and taking a screen shot ?

Mine works just fine but I cannot understand why you cannot. Sorry

Edit From what was said about cmd/shift/3 reply. Mine works with this as well....
 
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cmd + shift + 3 is screen shot


cmd + shift + 4 is selective screen shot
 
I'm having the exact same problem for taking screenshots while playing a game in full screen on a browser. It gives me a 26KB png that's just completely black. It works fine when I'm not in full screen. It's incredibly annoying. How do I fix this?
 
Can't take screenshots in Yosemite

If your looking to go third party, Skitch is a fantastic (an evernote product) tool, and also offers annotation.
 
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