the flash synched when you dry fired, was that shot at 1/500 like yu did the actual exposures?
because, obviously, the synch was off for your actual pictures. If the camera has a synch switch, make sure it is set on X. If it is, or the camera only has X-synch, then it is possible your synch is off. The synch for X should be so the flash fires when the blades are fully open -- if it is delayed even a titch, at 1/500 the blades will already be closed when the flash goes off.
So check that it is synching at 1/500. If not, then the tech needs to adjust that. Or you could just shoot flash at slower speeds.