Whether using TTL or flash photosensor, the flash will simply send light until the camera or photosensor says 'enough light' has come back from the subject. So if direct flash needed 50% of available power, it might need to send 100% of stored power with bounce, before it detects 'enough light'. But if the flash is maxed out with direct light at a certain distance, there is no way you'd ever get the same intensity of illumination at the same distance when using bounce!
Originally Posted by ted_smith
Now, all is not lost. Let us assume that with your subject at 15', using bounce the subject is underexposed by -2EV on that side of the face; that side of the face would be -2EV in brightness, but that is a GOOD thing! If f/8 output was available from the flash even with bounce, you'd have somewhat flat lighting that is too uniform across the entire face...not so good for flattering photos!