Key does not open rear door. on 2004 Honda Element

When I purchased my vehicle, I received two keys. One of the keys works in the ignition and all of the locks. The other key works in the ignition and most of the locks but not the rear door lock. What's the problem.

Could the second key be a valet key. Valet keys in other car manufacturers allow one key to lock and unlock everything and a second key will start the car and unlock the door but not open the glove box or trunk so you can lock your valuables in there.
Did the second key open and operate all the locks in the past?
Perhaps try putting some WD 40 in the locks and open and lock the locks with the "good" key then see if the second key now works.