Thanks for everything, everyone.
I will try the bigger box. The thing is, she's not just missing the box. She's going away to another part of the basement and pooping there. She leaves it in a nice pile, does it on the raw concrete (thankfully), right out in the open, and, it seems, still pees in the box (thank goodness).
Maybe pooping is harder to do in a box than peeing. I don't know (I haven't tried to do either thing
)
I had been cleaning the box daily, and she was still pooping elsewhere. It is pretty random. Now I've filled both of the old school boxes with litter (it's biodegradable clumping litter made of wheat). The other box has those non-flushable crystals (scilia?) that do a great odour absorption job. Problem is maybe she doesn't smell her scent in it.... I'm completely stumped.
BTW, I still can't figure out how to put a little picture beside my name.