Read vim official manual about event FocusGained (:h autocmd and go down to event list) and see what you can hope to get back on focus vim.
In real life, FocusGained event is working fine only for gvim.

Please, make it works also for vim from terminal (tmux, termite, urxvt and other... from any unix/Linux), or maybe if not able to do that, correct the manual to reflect the reality.