This happens for a number of reasons that I am aware of including 1) few search queries for that term, which often results in a one box 2) Google is mucking around with things...in some very competitive legal niches, you won't find local listings from time to time 3) Your term just simply doesn't trigger local, regardless of anything else.
I've worked a bit in the niche, and don't remember ever seeing a case where, for a typical term, it didn't trigger local.
Such is Google.