I'm also having this problem. I built a smallish house, with a loft room upstairs with a ladder and deck, but my sims won't take the dirty plates or laundry down stairs. I could build a staircase but that defeats the point of having a small house.
This also happens with laundry: If you have laundry on the second floor and the washing machine is on the first floor, the sim picks the laundry up and puts it instantly down again.
I ended up adding a staircase to my house because of that.
To add to the list of "things Sims won't carry up/down ladders" - I had a composting toilet on a different floor as the insect box. I forget the exact sequence of events now, but I remember a mess on the ground, then in the Sim's inventory, then making her wash her hands a few times afterwards.. The insects never did get fed.
