Take a photo in the same circumstances and settings, but without the flash firing.
If you can see an image, then you have ambient light contributing to your exposure, and this is the likely source of blur. Closing your aperture and/or lowering ISO will kill the ambient. It will also reduce the flash exposure on your image. This will need to be corrected by more efficient modifiers or by adding more lights, or increasing your flash power (which reduces action stopping power).
If you cannot see an image wihtout the flash, then try setting your flash to ACTION mode, and/or reduce the flash power (and increase ISO/ open your aperture). Either of these will reduce the flash duration, increasing the ability to stop action.
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