The iron key is rather… a special case when it comes to drops. I personally like the way it’s now, though currently the drop rate seems strange, at least for me. I’m not sure if I’m allowed to say more about the key… (I don’t work for the game or do anything like that, but I figured out some things about the key through some experiments and observations. And some tips by Rammy.)
The key generally only drops once a day, if you’re lucky 2 times a day from what I found out. But I can’t grantee that anymore, since the drop for me behaves strangely. 2 times now it’s been more than 24 hours without a key drop. It used to be within the 24 hour range before.