1 - Make sure you don't have your diffuser screen pulled down. This is easy to overlook and will produce that effect.
2 - Reset your flash - hold down the "Mode" and "On/off" button together for a few seconds. This gets you out of any funky mode you could have accidently put yourself in
3 - Put camera in fully auto mode. That clears any weird modes you could have gotten into there
4 - Clean the contacts on the flash and camera. I use a pencil eraser and wipe with alcohol and paper towel. Dirty contacts can cause flash misfire or cause full flash during the preflash mode, which gives you dim or dark photos or sporatic flash operation.

No proper sychronisation,or see menu select /iso/diaphram etc.