Have you looked through the lens while firing the flash? In a half dark room it is very easy to tell if the flash is syncing to the shutter in real time: crank the shutter speed up near the fastest speed, hook up the flash, look into the back of the lens (or the groundglass with a darkcloth and without a holder) and fire away. If you don't see the flash, it is going off to early or (more likely) too late. You can try using a significantly slower shutter, or send the shutter off for repair.
Hope that helps.