Hmm,
Let me work this out properly then as i agree it doesnt sound right after reading it again and your responce,
A worn key in a new lock will not lift the pins high enough to shear,
A worn lock with a new key would not lift the pins up high enough still,
A worn key in a worn lock wouldnt work
Hmmm, i think your right,
When on my duplicator i can cut 10 keys all perfect for differant locks but maybe one of them would be stiff and a quick brush with a file would let it work smoother, even though all the other keys work perfectly and all the locks are the same make and model the theorectical one that didnt fit would be too high,
I always put it down to worn locks but as you say its not so,
What is it then as calibration isnt the cause in my case,
Another idea while writing this for the OP could be that the spacing is slightly off as in the key wasnt set in the cutter properly maybe?