Try twiddling the steering whel a bit while operating the lock.
Next it could be grit in the lock? People tell you not to do this but I have found that WD40 sprayed into the lock and on the key ands then work the key in an out a few times will do it. DO NOT USE any other liquids, for install any kind of oil which IS bad for locks.
Finally a broken lock. Replace.
t, May 2007