Rebel XTI Flash Problems

I took some indoor pictures at work using the flash on the camera. On many of the pictures (at least 50%) there are under exposed areas (black really). It is not consistent at all. Sometimes it is the bottom of the picture, the sides or half the complete picture. I ahve attached 2 example pictures

They are taken from 1 meter away to 3 to 4 meters. There does not seem any consistency, except that most of them did not turn out.

The pictures were taken on Auto Program using an EFS18-55MM lenses

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I had the same issue with my G6, What happens is that your lens conceals the flash’s light
What you have to do to avoid it is to zoom in a bit.

I'd also asy to watch where your fingers are. I really can't think what else would have caused the shadows in the second photo - a lens shadow should be a nice arc out of the middle of the edge (typically the right edge in portrait orientation).

I agree that it's probably something blocking the light from the flash and casting a shadow. I've often been tripped up when I use the onboard flash and a lens hood.