The new hens are still babies, only four months old. We had to wait until they were almost full-grown so the established hen wouldn't injure them. She bosses them around, but for the most part they get along just fine.
The problem quickly became evident: no eggs. We went from an egg every other day or so from the older hen, to no eggs at all. The new hens were on the cusp of laying, but we waited for their contributions in vain.
Until last week, when we finally thought to check the top of the roost. Nothing up there but cleaning tools, chicken ointment, oyster shells... and eggs.
The one left there this morning illustrates what an inhospitable place it is for laying eggs, but that doesn't seem to bother them. I lined the nest box with fresh straw in the hope that they will switch. For now, we will have to keep the ladder in the coop to facilitate the search.