Since ubuntu 11.04 the scrollbars have been funny.
they are inspired by the osx lion scrollbars and only show when you hover over where they should be.
fortunately you can get the old ‘classic’ scrollbars back with a simple terminal command:
sudo echo "export LIBOVERLAY_SCROLLBAR=0" > /etc/X11/Xsession.d/80overlayscrollbars
once you have entered this command you need to restart your Xsession [restart your computer if you dont know how]
when you log back in you will find the old style classic scrollbars again!