I've noticed a problem on my new MBP (Late 2013) running OSX Yosemite 10.10.3 that caused this copy/paste problem, and other odd behavior in Outlook.
What I discovered was that if I clicked an object in PowerPower, and quickly copied to the clipboard (in less than a second), then the object would copy fine; but if I waited > 1 second, copy would not work.
Same is true of paste; i learned to very quickly copy/paste in under a second, and the function would work.
Not sure if it's related, but I also noticed that in Outlook on Mac, my selected email message would randomly change to the top of the inbox. I would repeatedly click down the list, and in a second, it would pop back to the top.
If the mouse event is indeed jumping off to some odd place after one second, then that could also be tricking PowerPoint into losing focus on the object copy/paste, causing it to fail.
I use a USB two button Logitech mouse, in case it's relevant.