Today I purged bumblebee and installed nvidia-prime again and found the solution.
The key is DO NOT REBOOT after changing video card in nvidia-settings. If you want to reboot right after the change, you MUST LOGOUT AND LOGIN then reboot, or, at least on my laptop, the discrete card will disappear from lspci and cannot be switched.
Purging nvidia-prime and a shutdown will recover the discrete card.
Today I purged bumblebee and installed nvidia-prime again and found the solution.
The key is DO NOT REBOOT after changing video card in nvidia-settings. If you want to reboot right after the change, you MUST LOGOUT AND LOGIN then reboot, or, at least on my laptop, the discrete card will disappear from lspci and cannot be switched.
Purging nvidia-prime and a shutdown will recover the discrete card.
nvidia-prime 0.6.2
Ubuntu 14.04
kernel 3.13-0-24-generic
nvidia-331 331.38-0ubuntu7