You’re not doing things wrong. The key detail (and it may sound a bit obvious) is that you need to reduce either the length (the side converging toward the right vanishing point) or the width (the side converging toward the left vanishing point) so those planes appear narrower. In your case, the cube at the bottom is slightly longer. If you adjust it by widening it a bit more, it will balance out, and the plane will appear narrower. What should you do? Ultimately, it’s about training your eye to spot these small details and learning to self-evaluate. After drawing based on a reference, place your drawing next to it and identify the mistakes you made (you can trace over the references if that helps you), and from there figure out the solution to correct them. Again, to draw narrower planes, try to avoid drawing very long lines, since that tends to widen the plane of the cube. On the other hand, just make sure your lines are converging toward the vanishing points. Hope this helps!
