The first lock I purchased to try picking was a master lock 141d, and I spent a full two weeks working on it with no results. I thought that perhaps I was simply not picking it properly until I got a Master 140 and picked it within two minutes, a feat I was able to reproduce with no trouble for the rest of the night.
So, what could be the problem? In the 141d, the third pin binds when tension is applied, but it never sets. And that's it. No other pins ever bind and there's no indication that the third pin has ever set.
It definitely isn't a tension problem; no amount of tension has led, in two weeks, to setting that pin.
And there's no indication of security pins. Pins 1, 2, and 4 retain their springiness. Pin 3, meanwhile, stays bound but never sticks.
The package boasts a "Precision Pin Tumbler Mechanism For Resistance To Picking". Could precise manufacturing tolerances be the culprit?