Use a faster shutter speed (whats the camera and sync speed?) Are you using natural light with the strobe? (Fill flash)
Anyway once you get the strobe set as the main source of light the shutter speed in effent becomes as fast as your strobe (my little 283 is 1/10000th of a second)
give use some more info and you should get a better answer
sorry I could not be of more help
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