Sorry, we still need more info.
Technically, you don't do Target Disk Mode with a "mini display port cable," you must have a Thunderbolt cable...but if it worked one way (rMBP --> mini) then it should logically work the other way.
As for your MBA...that isn't in your signature, so nobody here trying to help you can offer any useful advice.
So, once again, try writing exactly what you are using, and exactly what you are trying to do, and exactly what you did that a) did work, and b) didn't work.