Sims won't clean up dishes from high chair

A little more info... If there are dirty dishes on the table and on the high chair, then if I click a dirty dish on the table and select Clean Up, the Sim cleans up all the dirty dishes on the table, but ignores any dirty dish on the high chair. So it isn't just related to the dishwasher.

And if I click on the dish on the high chair and select Clean Up, then of course they'll clean up the dish on the high chair. And occasionally, some of my Sims will spontaneously clean up the dishes on the high chair, but not in the same trip as when they're cleaning up dishes on the table (or scattered elsewhere in the house).

HTH