Are you doing front laterals as well? That should help accentuate the tie-in between the anterior and lateral heads.
On the side laterals, check your form. I see a lot of guys that go super heavy, with ****ty form. Make sure that the angle between your forearm & upper arm remains consistant throughout the movement. At the top, look at how you're holding the weight in the mirror. Think of pouring a teapot

- your thumb side of your hands should be downward, and your pinky side of your hands should be upward.
You can try the movement now, with one arm, holding your opposite hand over the working shoulder. Perform the movement ending with your fist parrallel to the floor. At the top of the movement, pause, then notice the difference as you tilt the fist as stated above. With your feeling hand, you'll notice that the muscle tightens even more, right at the tie-in.
In practice, you'll do the twist with the movement, instead of only at the end. Same way you do dumbell curls, if you supinate.