Oh wow. That's... rather inscrutable to the player, unfortunately. Thanks.

I noticed this fast inserter was holding a wall and trying to place it in the assembler, which is paused because it's waiting for walls, so it has room to accept the item.

Mining the inserter (which added 1 wall to my inventory) and re-placing it caused it to start working properly.

How about a new-game-plus type of endgame? Build your factory, build your rocket... and get in it! Take the rocket and whatever supplies fit inside to a new planet, asteroid, or comet, with slightly different mechanics and challenges, different natives, different resources.