Maximum Shutter Speed & Band at bottom of frame

Playing around with some home studio lighting. Canon 5DII w/various lenses. All images shot at ISO 100, f/5.6, Shutter Speed 250. I thought the max shutter speed for flash was 250 but ALL my frames show a underexposed band at the bottom of the frame, or the side of the frame when I shot portrait orientation. I suspected a shutter issue and when I dropped the shutter speed to 200, the underexposed 'band' disappeared.

What do I need to know or learn?

PS This image has been processed and a vignette added...... so the corners have been darkened by me.

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