Cloudy finishes appear for several reasons, including using an old product or one that's been stored in a very cold or hot environment. Proper application also requires stirring the polyurethane coating immediately before placing it on the surface, which should be clean and oil-free.
Check that the polyurethane is completely dry, if you've refinished the piece recently.
Clear out the work area, and turn off any open flames or pilot lights nearby.
Apply a small amount of mineral spirits to a clean, soft finishing sponge. Wrap the sponge in a clean white cotton cloth. Twist the top to form a handle.
Rub the wrapped sponge back and forth on the finish to remove about the first coat. Let it dry, then load a clean sponge and cloth, and repeat. Do this until you've removed the blotching and finish and are down to the stain. Let the surface dry completely.
Lightly sand the piece with 220-grit sandpaper, if cloudiness persists. Sand until the polyurethane finish is removed. Reapply the finish per label directions, stirring the product as you work.