When I come across the word maxime in macronized texts, it usually lacks a macron over the first vowel. In Ørberg's Lingua Latīna series, however, in which the macrons are (from what I understand) very reliable, it's always macronized māximē. At first I thought that perhaps the macron was there to mark a vowel made long by position before the letter "x," but in other words with "x" (exercitus, mox, vexāvit, and so on) there's no macron.
So what's the deal? Is the vowel long by nature or not?