Say you take a picture, and say that the flash part of that picture is too dark. Like this:

There are two possible reasons, and it is important to distinguish between them, since they have two entirely different solutions.

1. The flash part is too dark because your camera and its TTL metering system metered the flash incorrectly, so it somehow decided on too low a flash power setting. This could happen because of subject brightness (a white subject); because you moved the camera; because of spot metering errors; because you set the camera to the wrong mode (it should be on “TTL”); or simply because of errors in the metering system.
2. There is simply not enough flash power available, given the chosen aperture and ISO settings.

So how do you know which one of these reasons is causing your dark flash picture?

Simple. Turn the flash, which is normally set to TTL, onto MANUAL mode, and select full power (100%, or 1/1). take the photo.

If you now see something like this, then the error was metering:

This means: given the selected aperture and ISO settings, there is enough power available, the camera just chose not to use it. The solution is to meter accurately, perhaps using matrix metering, or spot metering off a grey card; avoid recomposing; ensure that the connections are all clean and intact.

If, however, on full manual power you see the following, then there simply was not enough power available:

I.e. the camera would have liked to select more power, but it simply was not available. The solution in this case is to increase your ISO, or decrease your “f-number”, or bring the flash closer to the subject—or a combination of several or all of those.

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Many more tips when you take one of my courses, or read my e-books, especially “Pro Flash Manual”, from this page on the e-store.