Female apisto hanging out under the driftwood

If the female is not having active interactions with either the male or the other female I would guess that it is a health issue. If she was in pre spawn behavior I would expect a change in the interactions with the other Apistos. It is possible that she is working on claiming the cave, however, normally this would include some sort of "claiming" behavior which you don't seem to have.

I would suggest close observation for a few days.

DC

What would claiming behavior look like?

If she was "claiming" the spot I would expect her to try and keep other fish away, including the catfish. She would also be beginning to try to entice the male into coming her way by swimming up to him and rolling sideways to try and get his attention. This may well be without the yellow brood colors that we expect from females. It is not unusual for them to be quite dull prior to a spawn.

Without seeing the tank I can't say much more but I would add that generally a female that is preparing to spawn will seek out a very small cave. If this one is large enough that corys and plecos are using it as well I suspect that it is not prespawn behavior. However, if you don't offer small caves she will take what there is.

it's basically the only cave in the tank. She's still there as of this morning, and still sort of getting buffeted around by the cories. She ate the bloodworms that fell her way, though, so that's good. This weekend, I'll add another driftwood cave to give her an option.

I dont know what your driftwood cave looks like but it sounds like it might be a bit open and exposed. Try a small upturned pot maybe, to give her somewhere to get right out of the way. I built my two female Agassiziis their own cave each from two old camera film canisters (old school, I know!), it gives them somehwere quiet to get away from things.

Cut a v-shaped hole out of the lid, silicone the canister to a flat stone to stop it floating then cover the whole canister is silicone and roll it whilst still wet, in fine gravel. When it dries it will look natural and blend well. Mine have both used them to spawn within a week of the 'caves' being introduced.

The tank is heavily, heavily planted with rocks and driftwood and there are many places to "get away." I do not have an inch of bottom available to have built her a cave. And both females had been tolerant of each other up until now. Unfortunately, I can no longer find her. I assume she's not alive, but I can't find her body either and parameters haven't changed. Then again, I also thought all my amano shrimp and my otos had died as I couldn't find them for months. But last night, two otos were out grazing and the other day a shrimp looked me right in the face as I was trimming plants. So I hold out some hope she's still with us . . .