i have an XTi and a 420EX flash. the batteries on my flash ran out last night, so i took the flash off, and hit the button on the side of the camera to pop up the on-camera flash (AV mode) ... nothing happened. tried power cycling the camera, no dice. tried turning it to full-auto mode and taking a pic in a dark room, no dice. what could be happening? even if i pry the flash up with my fingers, it still doesn't fire when it really should.