Liftgate lock

I have a 98 Transport which I purchased used, recently I tried to unlock the liftgate with the ignition key (It is the only key I have) and it would not open (key would not go all the way in). Said key opens the driver, passenger and side door d/s but not the liftgate. Did the liftgate have with a different key or could the lock be jammed or dirty? If jammed or dirty how can I clean it out?

Thanks
Marty