Strobe not flashing
First, check to make sure that the transmitter is properly attached to your camera on the hot shoe and it's making good contact. Next, make sure that your camera settings are set up. Most cameras require a manual mode to be able to use the strobe. Also, check to make sure that your hot shoe was activated, some cameras have a separate setting for this.
Part of the picture is black
When using strobes, it is important to make sure that your shutter speed is not set too fast or too slow. If you’re starting to see random black bars appear or only half of the image, then it could be that your shutter speed is too fast. What is happening is that the flash is coming in while the shutter is still opening and closing since they are out of sync, the shutter itself is blocking your image reducing the shutter speed to approximately XXX and between XXX and XXX will help ensure that the strobe and your shutter is in sync.
Picture is not very clear when enlarged
Just like when your shutter is too fast it causes problems, so can having too slow A shutter speed can as well. Having too slow shutter speed allows extra light in and you wind up getting motion blur both from your handshake holding the camera and from the model or subject moving. Raising the shutter speed up to minimize ambient light yet capture the full strobe is the best way to get a good clear picture which is why setting your camera speed at your shutter speed at XXX to XXX helps ensure a good crisp picture.
Picture too bright or too dark
There are three main things you can control with flash photography. The first two are on your camera: Your aperture [F-stop] and your ISO or ASA which is your film speed bracket film. [What film]
Notice I didn’t mention shutter speed. As discussed before, your shutter needs to be set in such a way that it does not let too much ambient lighting but also does not go faster than the flash. In flash photography it is the flash Power that determines the amount of light that is coming to the camera. The shutter only needs to be open long enough for the flash to fully enter the camera.
First your aperture or F stop controls the amount of light able to come in through the lens. The higher the f-stop, the less light coming in through the lens. This is controlled by an iris that makes an opening larger or smaller depending on how you adjust the f-stop. Opening up the aperture wider, going to a lower f-stop, let’s in more light and thus you get a brighter picture. The same is true in reverse a higher f-stop number or smaller aperture results in a darker picture because more of the light is being blocked by the iris. Next is the ISO or ASA this is the film speed or also the light sensitivity of the photo electric cell. The lower the number the less light-sensitive enhance, the darker the image. However because you can control the amount of light coming from the strobe, you can shoot at a lower ISO number to achieve a finer quality picture then you can with higher ISO numbers, as increasing the light sensitivity also increases the noise in your picture. Last is the strobe power where you can adjust the power on the strobe to create more or less light during the flash. This allows you to adjust the brightness of your image without having to change the aperture or the ISO if you have them set where you need them to be. Effectively learning to use these three controls will help you take the best pictures possible.