During the Clock Arc in Fairy Tail, when the new Oracion Seis shows up, Byro (from Legion) was not able to nullify their magic. Why was he unable to do it? I don't think the activation depends on the opponent's power, and in any case if it does, he wouldn't be able to nullify Gildart's magic.
During the episode, after Byro is attacked by Midnight (or Brain II), Jackpot says that "Brain's magic refracted the range of Byro's magic and attacked him". This seems to tell us that Byro can only nullify magic within a range (it figures, as otherwise Byro would have been able to nullify all magic around him, however far apart it is, thereby preventing Natsu or Gildarts from even conjuring their powers). Thus Brain's attack lands on him as it surrounds him rather than coming in a straight line. Also, Brain attacked suddenly, catching Byro by surprise, thus giving an additional reason for Byro's inability to nullify Brain's magic. Then again, perhaps Byro may even have a limit to his ability, i.e. he can't nullify magic that is too powerful.
P.S: This particular incident happens in episode 140 'The Reborn Oracion Seis Appears!'. You may want to watch it again to see the part where Jackpot says the above quoted words. I have quoted the English translation that was shown on the screen, the perfect literal meaning may be different in different translations, so sorry if it's something else that you see there. I know only the anime version of the story, not the manga. Manga may have some other explanation.