All shots are actually not bad, just underexpose.
Try to use PS to adjust the brightness and contrast a bit and you will still have very nice pics.
How far away are you from the models ?? If you are too far, the flash might not be able to reach them. What's the largest aperture of your lens ?? Shoot at the largest aperture and set your ISO to 800 if need to. D50 noise level is still acceptable at ISO800. :bsmilie: