Can't use ground floor

I recently made a house, and my sims are unable to interact with anything on the ground floor level (including the surrounding areas that aren't part of the lot) and I have no idea why. If I tell them to use something on the ground floor they can't reach it, and I don't get a "go here" option when I click on the floor, but they are able to walk over it to reach things on other floors.

I'm only getting this issue on this lot, and I've tried adjusting the house's foundations which didn't help. The only other thing I can think of that might be causing it is different terrain levels. I tried flattening the lot and was told there was an item that couldn't be moved to the household inventory. (I very rarely use terrain manipulation so I don't know if this is normal at all.) I've now manually flattened everything except the edges of the lot, which are barely a different height, but it hasn't changed anything.

Does anyone have any idea where I'm going wrong here??