When this happens in my game I hunt around with the walls up and look for dishes my Sim can't reach. The last culprit was the new self from Tiny Living. I had it above the counters in the kitchen and sure enough there was a plate on it. I drug the plate down and now my Sim does the dishes. Later, I added a small plant to the shelf to avoid it in the future. Also, watch higher half walls. They put stuff up there but can't get it down. I think they also still have problems with the big coffee tables and big round tables unless that was fixed.
