Good game! One use plastics is an interesting take on the theme as a descriptor rather than mechanical function.
Like it's been mentioned before, but smaller items generally need to have bigger collision boxes to pick up at all, especially when the belts get faster and more numerous. The small size of the cursor's selection area is a little annoying too, it'll be nicer if it wasn't so precise I think, either that or today my dexterity is down.
Since we're dealing with physics objects there's not much we can do, maybe an invisible wall to prevent objects just falling off the belt and into a bin nearby which happened a fair few times.
Great job on the game!