Update: Ender-5 Plus is now actually printing
It looks as though I'm well on my way to having my first successful print on my Ender-5 Plus. I haven't gotten it to work on the two maze models that come on the memory card with the printer, but I did get it to work on a competitor's sample model (I may be overly optimistic -- it's not done yet). Weird. I haven't changed anything obvious in what I'm doing.
If you're curious, the model I'm working with is Lulzbot's Rocktopus.
Anyway, the original question still stands, at least in modified form: Why won't the maze models in particular print for me?