You appear to have a really solid method for attacking this lock. The only thing I can think of is that it's just a more aggressive bitting than what you're used to dealing with.
Are there any security pins? (Spools, serrated, spoorated, mushroom pins?) If so, this may be a contributing factor to your trouble.
Could it be that you're dropping pin stacks as you go? How about overlifting? This is a problem which I encounter quite often when picking anything like this.
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