Two more that fit your pattern of fine for weeks then gone. Calcium: they need it for the exoskeleton, so put a cuttlebone or crushed eggshell in there. And moulting: if one goes underground and disappears for weeks, that is normal and digging it up during a moult can kill it. Soft, vulnerable, no second chance.
Also check where your leaves come from. Pesticides, sprays, treated soil and scented products all do for them, so collect from untreated ground and freeze or bake the litter before it goes in.