Description:
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I have tested with your own examples and changing x and y 
parameters has no effect.

Reproduce code:
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$im = new Imagick( 'strawberry.jpg' );
$im->setImageFormat('png32');
$im->thumbnailImage( 200, null );
$shadow = $im->clone();
$shadow->setImageBackgroundColor( new ImagickPixel( 'black' ) );
$shadow->shadowImage( 40, 2, 30, 30 );
$shadow->compositeImage( $im, Imagick::COMPOSITE_OVER, 0, 0 );
header( "Content-Type: image/png" );
echo $shadow;

Expected result:
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Shadow should be far away.

Actual result:
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Shadow is still at approx 5,5 which I suppose are the 
defaults.