> I have been trying to let the camera TTL flash control do all the work.
TTL-controlled autoflash exposure is ok although of course you may have to bias it. What I meant (and one wonders why I didn't just say so) is to not let the camera run in a program mode in which it automatically sets the aperture. The camera may happily set a small aperture while the flash just doesn't have enough juice for that.
> You say to give about two stops extra exposure and you suggest doing this by going completely manual.
No, you shouldn't have to give the extra exposure; the camera's ttl flash control should take care of that _assuming the flash has enough power for the aperture in use_.
Let me give you a few examples; I use a Sunpak 1600A, a couple of Metz 32-Z2s and a Metz 45, all with bounce flash. It's extremely rare that I've ever used direct flash. The Sunpak is a tiny low-power thing about half the size of a cigarette pack, the Metz 32 has about the same power as a Vivitar 283/285 or Nikon SB-24, and the Metz 45 has about a stop more power.
Using the little Sunpak and EI 400 film I can confidently shoot bounce flash in ordinary home-sized rooms using autoexposure at f2.8 Although the flash has settings for smaller apertures and those can be used in direct flash it simply doesn't have enough output to use the smaller apertures for bounce flash regardless of the fact that they can be set. The Metz 32 runs at f5.6 and the Metz 45 at f8. I simply set the lens aperture setting appropriate to the flash I'm using and let it rip in auto...perhaps with a minor exposure bias.
The point of all that is that to do bounce flash you must use a film speed and aperture setting that's appropriate to the flash you're using. If you're consistently getting dramatic underexposure then most likely the flash isn't powerful enough to operate with the aperture you've set and/or the speed film you're using.
You can (and should) let the flash run in TTL, but if you're letting the camera pick its aperture you may simply need to switch modes so you can set the aperture directly to something that'll likely work, such as f2.8 or f4. If your lens is a slow f5.6 zoom and you can't set a wider aperture then you'll need either a more-powerful flash or faster film.
Most likely an exposure-compensation adjustment will be needed, perhaps up to +1, to turn a maybe-adequate TTL-controlled exposure into a good TTL-controlled exposure. This is normal; in fact the exposure-control circuits for continuous light and flash are often separate and have separate sensors. While the results should be the same they rarely are. But at any rate, while a TTL-controlled autoflash exposure may be slightly less than a continuous-light exposure it definitely shouldn't be dramatically less whether the flash is direct, bounced off a gizmo, bounced off your white shirt, the floor or the ceiling. If it is, that indicates to me too small a lens aperture and/or too low speed film being used for the available flash power.