Which mode are you using?
If you are using aperture priority then there can be a tendancy for the camera to under-expose the image (not enough power in the flash for the given aperture).
Programme mode is the simplest and generally gives properly exposed images, though no control and under-exposed backgrounds.
Shutter Priority mode is useful to expose correctly for the ambient light with additional fill in flash, however, if there is insufficient ambient light you will end up with a very narrow depth of field and/or need to use a very very slow shutter speed.
It all depends upon how you have the camera set up, what mode you are using, etc...