Why doesnt this script work? (marginLeft and marginRight)

JavaScript
1

This is working fine:

	    // left
		$('.left p')
		    .bind('enterviewport', function() {
		        $(this).animate({
		            opacity: 1,
		            marginLeft: 0
		        }, 400, function() {
		          // animation complete
		        });
		    })
		    .bullseye();

		$('.left p')
		    .bind('leaveviewport', function() {
		        $(this).animate({
		            opacity: 0,
		            marginLeft: '-100px'
		        }, 400, function() {
		          // animation complete
		        });
		    })
		    .bullseye();

But this isnt working at all:

	    // right
		$('.right p')
		    .bind('enterviewport', function() {
		        $(this).animate({
		            opacity: 1,
		            marginRight: 0
		        }, 400, function() {
		          // animation complete
		        });
		    })
		    .bullseye();

		$('.right p')
		    .bind('leaveviewport', function() {
		        $(this).animate({
		            opacity: 0,
		            marginRight: '-100px'
		        }, 400, function() {
		          // animation complete
		        });
		    })
		    .bullseye();

And the only difference is the not working one uses “marginRight” instead of “marginLeft” … anyone can see why?