So, as I posted in my last message above, I think this is due to blocky stairs. You can see in the attached video that the building I design between 12:46 and 17:04 (stairs between 15:50 and 16:51) does not get built when I order it built at 17:25 (still hadn’t been started at 18:43). I redesign an almost identical building but without stairs between 19:43 and 21:18 and have it built at 21:21. The workers start that one immediately.
I think it’s either the block tool itself that is causing the problem, or the fact that I placed some blocks in the same place where a wall would also be. I forgot about that because I was using the view that cut away walls where the camera is looking.