Yeah, you need to mount the ball so that the motor's axis stays fixed in one of two orientations. If the ball can move around, thereby "moving" the motor's axis of rotation, you won't make more or "better" motion, but you will have a hard-plastic ball bouncing around your plastic decoy, causing noise. This constant noise will definitely will annoy you, and may even spook ducks--a sight that can make grown men cry. It will also hurt your battery life. All bad things.
So, let's get back to the two motor axis orientations that will be the best options for duck hunters . . .
(1) vertical axis orientation - In this orientation, the counter-weight on the motor (i.e., the AA battery and lead, bolt-on cover) tends to cause your deek to have a left-right-left oscillation. This position has multiple benefits. First, I've found that this position makes the most wake in your spread (and no, it's not too much). Second, because the motor does not have to "lift" and "lower" the attached counterweight, the motor has to do less work, improving battery life. This will usually be your best bet.
(2) horizontal axis orientation - If you choose this route, the counter-weight will tend to cause your deek to oscillate (or "bob") in a front-back-front fashion (Note: the counter-weight should be moving toward the front of the deek at the top of its rotation, so that the motor will also tend to cause your decoy to move slowly forward, not backwards, for a more life-like presentation). Because your deek is longer than it is wide, this orientation produces less wake than the first option, but it's still a respectable amount. You'll also be buying more batteries, because the counter-weight must rotate in vertical directions.
But, it produces great, life-like motion on surface feeder-type deeks--your deek's beak will bob in and out of the water as it slowly moves around in the water!