Debian/Ubuntu do not have bash autocompletion enabled for root account by default. When working under root account, this is quite missed feature, but luckily it can be enabled very easily:
cd ~
apt-get install bash-completion # if not installed - like in minimal Debian install
nano .bashrc
add folowing to the file:
if [ -f /etc/bash_completion ]; then
. /etc/bash_completion
fi