2009 Ford F350 drivers side door will not lock..

The drivers side door will not lock manually it just pops back into the unlock
position.  I can get it to lock for a second with the key but when I move the key
back to take it out the door unlocks again before I can take the key out.  I have
checked all the other doors and they are fine, they make a kind of click sound
that let's me know they are locking and unlocking, but the drivers side door is
not doing this.  Any reason for this?   

Thanks, Adam.