Getting your projectiles stuffed tight enough can be difficult. Often, it comes down to you jamming one bean at a time into the last stitching hole before you close. But here's a trick. Twist stitch the last two inches or so a bit loose, then stuff the ball tight as usual. Right before the final closing knots you can go back and pull both sides of the twist-stitch tight. This will give you a bit more slack for the closing knots and pull the halves together giving you a nice solid ball to sling with.