Did you lift them up a couple of inches. Both my 2005 and 2007 have a lip on the back that slips over a lip attached to the wheel well.
To take off I take out the screws and tap the cover in the middle of the bottom with the heel of my hand until the come off the lip. Dirt and crud do act like an adhesive but they break free if you go straight up. Reverse to reattach.
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