If you have the setting set clipboard=unnamedplus
in your .vimrc
then this will not be working.
For OSX you have to use set clipboard=unnamed
For Linux you will probably need to use set clipboard=unnamedplus
Heres the snippet from my personal .vimrc
if system('uname -s') == "Darwin\n"
set clipboard=unnamed "OSX
else
set clipboard=unnamedplus "Linux
endif