It seems that a bunch of them spawn on the right side? I looked at your code and believe I have found the problem: "addObject(coming, Greenfoot.getRandomNumber(490) + 5, 5);" should just be "addObject(coming, Greenfoot.getRandomNumber(400), 5);"

Actually, I didn't use loops for moving up and down. That doesn't work in Greenfoot, because it all happens w/i one frame. I have a counter that starts at -10 and goes to 10. When the counter is negative, it goes down 1, when it's positive, it goes up. I'm going to be working on scrolling soon. I haven't started it yet, but I hope to include it in the next update. I wanted to start with a couple different type of enemies before I started working on scrolling.