Thank you Fletch, I am happy you liked it! The merger needs the two parts from two different belts, it should merge them even if they would overlap. If you tried that way then please post a screenshot so I can try to find the problem.
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.