I'm not familiar with that particular computer, but laptops that I've come across don't need any switch to be actuated, in order to swap from mains to battery power.
Normally you just plug in the battery and unplug the mains.
Are you sure that you have the battery fully plugged in? Could there be a dodgy connection to the battery connector?
And have you checked the battery voltage to be absolutely certain that it is holding a charge?