I think, the belt coming with the box should also go to the right, into the merger. If you had the belts going in other directions before it might be possible that a green paint is in the upper slot and the box should go to the bottom one (I wanted to add an indicator for occupied slots, I'll revisit this*), or that the paint is stuck somewhere in the middle, though I could not reproduce the issue (I will update it to have the component snap to a slot*)... Could you please start over and connect them in straight lines? I hope that solves the issue.
* I will keep the jam version as is and will post a separate game with the updates.