It don't have nothing to do with the key. If the key turns the ignition, it should turn the lock.
Check if the peg is lining up with the hole. If the bolts loosened a little bit, it'll be hitting below the hole.
Seems like the peg is hitting on somehting. OR- the lock could be busted somehow, from someone trying to steal it or somehting. May need a new ignition. Who knows?
Definately not a key issue, though.
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