Christmas cactus/group.
They all seem to get really thin flat leaves when I have them awhile. And the pot becomes very dense with tiny roots? I just don't know how others get them to have nice plump leaves. Current one has been out in the shade all summer and never got happier, sadder, anything.
My MIL has a little three inch potted grocery store CC. Never repotted in 4 years. Nice and plump. I water about weekly, it runs over the side of the rootball and leaves a tiny bit in a bowl underneath. Loves it.