[jfeenin]

Some things I can think of are:

- The flash cannot put out enough light for the given situation. (Ceiling too high, room too large, walls or ceiling too dark to bounce effectively, distance to subject too long, focal length too long etc.)
- Some other light source in the scene or some other factor is confusing the camera''s metering of the flash.
- Flash is obstructed by something you don't notice at the time.

Some things I would try: (try to replicate a situation/scene for these tests that mimics one that normally underexposes).
- Increase the ISO, this will require less flash for a given shot and if there isn't enough flash power it will allow for it.
- Try manual mode f5.6 or lower at 1/125 or lower and take the same shots. Try to let the flash do all of the adjusting and force the camera to a fixed shutter and aperture. Try difference ones and see what the results are.

Remember in camera metering for exposure and flash is getting better and better. These days it is pretty good but there are still times it will fail. You may just be consistently taking photos in situations that don't lend themselves to that. A hand held meter would do wonders in diagnosing that if you can borrow one from someone near you to test.