hi,
i think if you are not able to detect a drive in that slot, even when the same drives work in other slots, then it could mean that the backplane has been damaged.
am not sure if you move the drobo around, or if you purchased it 2nd hand (bearing in mind its age) but it might have been damaged. if you got it brand new from a shop though, you might want to take it back and show them to get a replacement or refund, as im not sure if Drobo support will still support it?