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